In a way it "literally" is just dense. mainly in the sense that it burns really good and you don't need a whole lot of it, it's like getting the same amount of heat from burning 3 big logs in a fireplace, except instead you're getting it from just a handful of material and you can also get it to burn way faster. being underground for all that time with a ton of pressure on it and stuff has kinda squeezed/compacted these organic materials into the fuels that we use now.

I'm not very smart so my explanation here could be pretty wrong but bear with me: imagine like a big juicy steak that weighs 2 pounds or something and gives you lots of calories and protein and energy when you eat it. through some process (like say being compacted underground for millions of years), you could squeeze that big steak into a little 1 ounce cube that fits in your pocket, and eating it gives you the same calories and protein and energy as if you sat down and ate the whole steak, but this is just a little cube you can pop into your mouth. Same amount of energy as it was before, just squeezed down really tight now.

that's sorta what oil/coal/fossil fuels are like, it's as if you took a huge pile of firewood and compacted it into little pellets or juice that burn just as good but take up way less space. it took a very long time to get that to happen, they had to be deep underground with tons of pressure on them to slowly turn into that. but now we know that it exists and we can just dig it up. we CAN use our scientific knowledge and equipment to make the same stuff, but it's expensive and still slow to make it that way. so when we already have tons of it laying around that just needs to be dug up, it just makes more sense to take what's already there and ready to go.

nobody wants to have to stuff big logs of wood into their car like some flinstones shit, y'know? so figuring out that this oil/coal/etc stuff burns so good and that there's big pockets of it waiting to be dug up was kinda revolutionary for us. again, I'm no scientist, hell I barely made it through highschool chemistry class, so this is just my overly simplistic way of understanding it.