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.