Hello! I kinda thought this was more of a rules question, but when I reviewed the pinned post on policies for that topic, I found this:
If your question is "why is it that way?" or "why can't we do it this way?", it's a philosophy question rather than a rules question, and you should be posting in Ask The Writers instead.
And this is, in fact, a "why is it that way?" question.
So, getting right to it.
We know that humanoid 'mechs have articulate waists. Even setting aside the numerous instances in the fiction of 'mechs bowing and so on, the fact that there are rules for 'mechs bending over to pick up cargo establishes this.
Therefore, why does a humanoid 'mech need to have at least one arm to fire from a prone position (propping itself up, essentially)? Why can't it just sit up? I mean, unless they have the proportional strength of the 50 pound bag of dried peas that mirrors
my core strength it seems like it should be a snap. (Related, I bet you anything there are fit one-legged people who can stand up without using their arms, but I won't go that far.)
Cheers!