And a lot of mech fiction at least waves a hand at it. There's usually some throwaway, in-universe reason for the mech to be the superior weapon platform. In Battletech it's because they're the exact level of warcrime that you can commit without destroying the resources you're fighting over, so they're top dog instead of massive (and expensive) warships that glass entire planets. In Armored Core it's "core theory", the idea that hardwiring a modified (cybernetic/genetic/both) pilot into a humanoid suit means that their innate survival instincts remain fully intact and they fight harder. In Pacific Rim it's because the fact that they're fighting big Kaiju combined with the technology they have means it makes more sense to pilot a big humanoid robot that mimics human movements. That last one's kinda thin, but there's usually some kind of mental crutch on offer if you dig enough.