No need. Solasta does it; the controls are just a little bit awkward, but once you got used to them, there is no problem.
XCOM does it as well. Back in 99 Howeworld has whole real time RTS in 3D space.
The issue isn’t that flying would be ground shaking tech - maybe they didn’t think of it, or didn’t think it was a good idea, or it would diverge too much from their D:OS2 formula, afterall flying in the air would cancel a big chunk of Larian’s home brew - pushes gone, guaranteed to hit surfaces gone.
