Starfield's big problem is it's a huge universe built on an engine that really can't support massive worlds like that. The reason you can't fly around on the surface of a planet is because their crappy engine can't cope with that much space existing, and it can't load more environment when you get to the edges like every other game does because their engine doesn't support proper level streaming.

If you mod the game to force the issue it gets glitchy very very quickly.