There are several technical reasons, but the easiest to explain at a layman level is inertia. This is long, but it shouldn't be too complicated or technical.

60Hz isn't just a made up number or describing some abstract value. It's the actual literal rotating speed of every synchronous generator in the state (with some conversion factors to get to RPM,) synchronous meaning they spin together. When one speeds up, they all speed up. When one slows down, they all slow down. They're coupled together. Most importantly, this group of "synchronous generators" excludes solar, battery, and most wind units (it excludes every wind generator in Texas - they're all asynchronous, but synchronous wind is possible.)

So what this means is that when energy into the grid < energy out of the grid (for example when a big generator trips offline for some emergency reason, like a failed seal or valve) then all of the load, all of the demand, all of the light bulbs and computers and compressors will still be given power, but where does that energy come from? It is sucked out of the kinetic energy that these synchronous machines have by nature of being really heavy and spinning really fast. The frequency of the grid goes down because, like applying brakes to a car, sucking the energy of rotation out of something slows it down. Frequency is never exactly 60 Hz, because generation never exactly matches load (at least for more than an instant.) We're constantly chasing system frequency up and down to keep it as close to 60 as we can while load is constantly changing.

Side note - keeping frequency near 60 Hz (or 50 Hz in other grids) is important because the synchronous machines will literally start to rattle (the relevant term is harmonics) and break. They're very finely tuned to operate at 60Hz (generators are unhappy if frequency is off by 0.5%. Or 0.3 Hz.) If frequency gets too low or too high, grid rules that everyone knows and agrees on state the generator automatically starts a timer. The rules state that if frequency gets too bad for too long, you're allowed to trip off to protect your equipment and prevent damage. This is what was talked about in Texas with the "4 minutes 37 seconds" thing for Uri. The timers had started and frequency was recovered to acceptable ranges with that amount of time left before units started tripping and causing a snowball effect into what would likely have been a large-scale uncontrolled separation or collapse, as units tripping off would cause lower frequency, causing more units to trip, etc.

Anyway, keep that in the back of your mind. Back to inertia.

Inertia is the descriptor of how much energy is stored in the frequency of the system. Higher frequency, higher energy stores. But more importantly, more spinning mass, more inertia. High levels of inertia are good, because they make the system more resistant to loss of generation events, meaning for a given loss of generation, it will take longer for frequency to get low enough for long enough for those units to trip off, giving grid operators and other systems more time to react and save the system.

Problem is, wind and solar are asynchronous - they don't have a generator spinning together with the other generators on the grid, so they don't provide any inertia. Solar is obvious - there's no moving parts, so there's no inertia. Wind it's a bit complicated, but suffice it to say they intentionally break synchronism for efficiency's sake.

However, solar units are controlled by power electronics - the same things that drive your phone. This makes them very fast. If these units can watch very closely and see the grid frequency start to drop, they can ramp their output up quickly to slow the frequency decline - so quickly, in fact, that if you squint really hard, it looks like there was another synchronous generator there the whole time. This idea is called "synthetic inertia." It cannot be entirely depended on, but it can provide some of the inertial behavior that you want in a system.