They can't because asians have been mind washed to believe that you must achieve a certain velocity to go into space and another velocity to leave the Earth's sphere of influence, wherever the boundary might be. And I'm guessing the reasoning here is that Gundams aren't aerodynamic enough to go hypersonic. From what we have learned in the first season (because s2 is a bunch of incoherent unicorn dump), the GN drive is reactionless and works by manipulating gravity, that's why it has no thrust and its pilots are unaffected by the tremendous manoeuvrability. We don't know the exact power output of these things because they can also work off capacitors but we can safely assume that they can maintain flight indefinitely, so >=9.8m/s for any duration. That is more than enough to go in orbit from the surface, without even pushing up the throttle, and definitely no need for "Trans-AM". Another reason it's a bunch of incoherent unicorn poo-poo. Something else might be, how the hell do you square your power output? If you measure your power in kW the result will be 1,000,000 times larger than if you measure your power in watts. And how about the fact that the 00 Gundam doesn't seem to have ANY performance advantage over other single drive or even fake drive MS after the second episode?