The human mecha designers and engineers weren't actually bad.
The fact that they actually managed to create military grade two hundred meter tall metal behemoths capable of flexible fighting like a real human being bore testament to that.
It was just that we were a very young race.
We did not have the accumulations of the aliens which had millions of years under their belt, and strings of failures that would beggar a billion earths.
And without failures and time, how would we learn?
And so the aliens did it much better than us in terms of mechas.
Design objectives that we needed three to four big devices to accomplish were accomplished in one tiny device, while layers upon layers of insulation and the sort were discarded and replaced by only a thin layer of cream-like substance, a result of ingenious elemental manipulation that surpassed anything our smartest and craziest minds could imagine.
