Plenty of space games have real Newtonian physics. It's one the simplest thing to do as far as realism in gaming. Even Asteroids has more realistic space flight.

The real reason is the engine does not handle player speeds very well. It never has. If you notice, you can't make your max speed higher than 150, and your boost speed is only double that for a very brief period of time. The only reason I think you're able to even go faster is because there's no world to fall through in space. There's barely anything rendering to be affected by the speed. If you were to try going super fast in Skyrim or Fallout, you can fall through the world as you start going faster than it can render, and it doesn't even take that much speed to hit that point.

Elite Dangerous does. I forget what they're called, but there's a toggle keybind that lets you turn the stabilizers or whatever off so that if you throttle up and then cut the throttle you'll continue moving infinitely in that direction until you turn the stabilizers back on or thrust in a different direction.

It lets you do wildly badass shit like boosting past someone, cutting throttle and stabilizers, spinning around, and blasting them as you fly backwards and they're still trying to turn around.

It's not exactly difficult to program. The only reason they would have done it this way is because they think it feels better to control. Realistic space physics results in a lot of crashing into things, or more commonly flying past them, which can result in frustration.

Outer wilds had this. I often die due to speeding up too much hahahah.