Because it’s really really hard to become a test pilot from her known background.
From what we know… it’s like saying, “Yeah I just learned how to bike, now I can fly that military Gundam.”
Even if she knew half the higher ups and served a few years in the military…
I think it would be more likely if she stole the plane, then that she tested an experiential one.
(Edit: Look the only way this would make sense…
If Tracer was actually “Royalty”.
A direct member of the “British Royal Family”.)