Everyone knows that in a fantasy world, the less your armor covers the better it protects. So the reason you won't find mechas is not because of some technological level bullshit, but rather the logical conclusion of mechas providing too much defence compared to how much clothing you can remove. Here we see the solution: wear a mankini and you can indeed equip a mecha, though it has to be the open-design kind of mecha, both to keep the defence-stat low enough to fit inside the lack of clothes, and to not end up covering stuff up and as such return to bikini-armor's "ludicrously high" amounts of coverage.
TIL