By fiction vs non fiction. By the time you're capable of manufacturing energy devices capable of powering a 'mech', you've already been manufacturing von neumann starseeding/mining probes for a long time, and those probes will of course be of exponential mass, equivalent to and quickly exceeding entire star systems, carrying sentient AI fully capable of crashing rocks, planets, even stars and eventually blackholes into targets, obliterating anything worth engaging like a sad game of billiards.

Sorry to kill the discussion with the reality of it, but gundams are silly ass kid shit