Humanoid mecha are the kings of the genre, both Real and Super, for one reason: it allows you to depict human action without concern over depicting the consequence of that action. It's why humanoid mecha routinely become a character unto themselves in a way that non-human mecha do not, and media producers (and their friends making the merchandise) are well aware of that fact.
The most obvious example of this fact in action is the routine depiction of violence upon a humanoid mech in ways that, if it were a person, would result in howls of outrage from Concerned Citizen astroturf groups. (Yes, this is why no one died in the 80s G.I. Joe cartoon unless the plot literally demanded it, and why the TMNT cartoon had robot ninja in it.)
Instead, you can get away with a brief show of the doomed pilot and a flash-to-white as they died.
That Super Robots assumed this form as a matter of dogma isn't surprising when you see the robot as a substitute for man-to-man action, for the above reasons, and it shows in how the early ones were not that different from the Fighting Man heroes that their pilots clearly are descended from.
Put simply, you can trace a direct line from Beowulf to Koji Kabuto (that will run through John Carter and Kimball Kinneson), and if the former got moved to the latter's milieu it wouldn't take the man long to become a competent Mazinger pilot. That carries over to the Real Robot side, albeit often more subdued these days, but Ace Pilot is a contemporary Fighting Man archetype for a good reason- and it exists in the real world.
This is why mecha shows so often stick with a humanoid unit for their heroes and villains, even when otherwise they'd be in a GERWALK or other non-human unit (e.g a four-legged unit, such as those of Fang of the Sun Dougram), and those that do vary from this norm tend to be the ones that aren't as successful.
You definitely see this influence in original designs for properties such as Super Robot Wars, and even BattleTech)--among the realist of Real Robots--has humanoid designs for its top-tier figures- yes, even Clanner scum. It should be no surprise to you then that my own Star Knight Saga follows this norm for my hero, his allies, and his enemies-with a justified reason for why those humanoid mecha exist.
I think a lot of people who do mecha fiction forget the roots that give substance to the fiction they're writing, and in this case it is vital to remember the heroic roots of giant robot shows, comics, and prose. When you forget, you get forgetable dreck- or worse.
I have not forgotten.