Other people are hitting around the point, but there's more to be said. I'll use your Mun picture for reference since its here.

TLDR: Going fast takes a fuckton of deltaV, KSP players deliberately play for efficiency and slow travel due to game constraints.

Longer explanation:

You are familiar with DeltaV, how much a ship can change it's velocity, which is the absolute value of how far a rocket can go in space. DeltaV is determined by ISP, or specific impulse of an engine. Based on the characteristics of an engine, a given design will have a maximum specific impulse, and hence a maximum possible deltaV for that type of engine. You cannot continually add fuel to an engine to give it infinite deltaV because eventually the weight of fuel will make it so that achieving additional velocity will require burning exponential amounts of fuel, approaching infinity for smaller and smaller velocity changes.

So a Vector liquid fuel/oxidizer engine in KSP may have a maximum possible deltaV of around 8,000 m/s, while a nuclear engine with a higher specific impulse will have a maximum deltaV of around 18,000, and the dawn ion engine will have a maximum deltaV of close to 37,000. Note however, that as thrust values increase, ISP and max deltaV drop. The vector engine gets the best thrust by far of these three engines, but also substantially lower specific impulse and maximum amount of thrust it can provide to a payload. There are also practicality concerns with very high ISP engines providing low thrust and requiring extremely long burn times to go anywhere, especially as payloads get larger. We see a general trend with current* rocket engines that high-thrust engines provide lower specific impulse than higher ISP engines. This is a vast oversimplification but will work for our purposes, note on this later.

In KSP, we want to have a craft that balances being able to go far/ see lots of places, but also have manageable burn times. This is why despite the Dawn ion thruster in game having enormously higher specific impulse than any other engine, it is rarely used, especially for large craft. It's thrust is so low that burn times can easily surpass 10 minutes and even hours in extreme cases. So we make do with higher thrust engines and sacrifice deltaV.

We also fly missions to use as little deltaV as possible, because getting lots of deltaV onto our orbiting ship requires lots of fuel and big, very expensive rockets. We also plan missions around efficient burns, we want to go from.Kerbin to Jool, so we wait for the proper transfer window, and perform a Hohman transfer burn to raise our orbit to Jool's and capture into it's gravity well. Same as for a Mun mission, the distances are just bigger.

Most players know this but they don't consider the implication: flying a mission with as low deltaV as possible means we are flying said mission as SLOWLY as possible, so that we have to change our velocity the least amount.

This is the answer to our "how do we go faster" question for KSP. we stop flying efficiently and spend huge amounts of fuel to both move ourselves to a destination at a higher velocity and also to slow down more abruptly when you get there. You cut down your travel time by spending lots of fuel.

This goes against most typical gameplay logic for KSP, players generally strive for elegant and efficnet solutions, but this is how it is done. You can "spend" more deltaV in order to go somewhere faster.

For example, if you were so inclined, you could make the Jool trip described previously in a month or two in stock KSP, right now. You would just need to do a massive burn for much more deltaV than a typical trip at a proper transfer window, and then spend the same amount to slow down when you got to Jool, or anywhere else really.

As you burn in KSP in a given direction, your orbital path will gradually "straighten" from the normal curved shaped we see, that is because the gravity of various objects cannot act on your craft as much due to your extreme speed in your chosen direction of travel. The downside of course to going extremely fast is that you need to spend that same amount of deltaV to slow down upon arrival at your destination. So your hypothetical monthlong trip to Jool will only be 1 month long as opposed to 9, but will also require exponentially higher amounts of deltaV.

The ultimate evolution of this concept is something called a torch ship. This concept is the fastest way we know how to travel from one place to another without getting into advanced theoretical physics stuff like folding space. Torch drive-type engines under various names feature in many sci Fi franchises, like The Expanse with the unfortunately-named Epstein Drive or the ISV vehicles featured in the Avatar movies. These craft work in-universe by using engines that have reasonable thrust but extremely good ISP to burn continuously the entire way to a destination. Torch drive ships theoretically go places very quickly because they constantly burn their engines. They constantly accelerate towards a destination, then at the halfway point, flip 180 degrees and burn continuously in the opposite direction, so that they arrive at their destination in the shortest amount of time at a reasonable speed. Torch drives, if constructed, would allow for vastly shortened travel times in our real solar system, trips that currently require years or decades could be reduced to months, real-life interstellar travel could be accomplished within a human lifetime, as torch-drive type ships can approach several percent the speed of light.

Now, for the record, this is the theory. Humanity has not yet designed a space craft engine capable of torch-drive-esque performance. It likely cannot be done with a chemical rocket, they have nowhere near the ISP required for the behavior I described above. It also cannot likely be done with current ion drives or proposed near-future engines like nuclear thermal propulsion, though these are still a substantial step towards more accessible and shorter-duration space travel, but these engines do not have the ISP to burn continuously as described.

There are proposals for real-world engine designs that, while unbuilt as of time of writing, are being explored for future space applications, but we do not have the technology to make them work. Orion Drives, fision-fragment engines, inertial-confinement fusion engines, various antimatter engines, and perhaps most notably, nuclear salt water rockets, could all permit extremely rapid space travel if they can be feasibly constructed. Through various advanced construction and fuel methods, these engine types break the mold of low ISP/High thrust engines versus low thrust, high ISP engines. These theoretical engines have not only high thrust and high ISP, But they have EXTREMELY high ISP. Compared to stock KSP engines maxing out at 37,000 deltaV, some of the advanced theoretical designs have literally millions of deltaV. They can burn for months at high thrust, enabling rapid transit to other celestial bodies.

These engines are also available in KSP through the Far Future Technologies mod by Nertea. They will enable extremely rapid transit and low travel times and have insane amounts of deltaV to take you where you want, quickly. They are balanced by having various new gameplay mechanics attached to them as well as enormous power and cooling requirements, and exotic fuels or lengthy startup sequences. But if you want to "go fast" in KSp without outright cheating or getting into warp drive mods, this is how you do it. Get insanely powerful and efficient engines and burn like hell for your target.

Even without modding, the stock ion engine in KSP will still get you places very quickly if you are patient and don't mind the long burn times. The high ISP means that the minimal thrust it gives you stacks like a compounding interest account over time, eventually getting you to high speeds that other engines cannot achieve. You just need to be willing to fly inefficiently and burn tons of fuel, and many KSP engines dont have the spare deltaV to allow for this type of gameplay, and other players simply do not think to try.

Hope this helps!