My headcanon is that the Mako has to drive on every kind of terrain, in every kind of gravity, so the best anyone can do is make a machine that drives across all of them…. badly.
Also, my headcanon of in game character conversations:
Shepard: *grits teeth to avoid next pile of rocks*
Garrus: CAN YOU JUST NOT DRIVE? IS THAT THE PROBLEM? CAN YOU EVEN DRIVE AT ALL?
Shepard: I. Am Trying. To. Drive.
Garrus: YOU ARE FAILING COMPLETELY
Shepard: *runs over suspicious looking cow* Fine. You take the wheel, if you think it’s so easy.
Garrus: *Runs the Mako straight off a cliff* AAAARRRGGH
Shepard: Told you.
Hence why Garrus spends all his downtime with the Mako: He’s trying to modify it to make it handle like something you can actually drive.