Because that’s around what they’ll experience in flight. [Check out the Wikipedia page on g-loc.](https://en.m.wikipedia.org/wiki/G-LOC) An untrained person will pass out between 4 and 6 g. With the right equipment and training pilots can sustain close to 10 g for brief periods. So it makes sense for them to train at around what they will experience in in flight. The guy who does XKCD wrote a post about “[no-rules NASCAR](https://what-if.xkcd.com/116/)” where he discovers that the human body quickly become the weakest link if we try to design a fast accelerating contraption.