My headcanon is that mechs require a few things to be a viable weapons platform:

1. Orbital bombardment is not a viable tactic
2. FTL travel is cheap and easy for non-living matter
3. FTL travel is expensive and/or prohibitively dangerous for living tissue
4. Artificial intelligence / fly by wire is not viable

If 1 isn't true, then toss rocks at them from space and pick up the pieces later.

If 2 isn't true, then it would be easier to train local forces and use commando teams

If 3 isn't true, then it would be cheaper, easier, and more effective to deploy rapid response forces of mixed armored infantry.

If 4 isn't true, then send your swarms of autonomous weapons platforms to kill anything that moves.

I'm sure there's a few other reasons why Urbanmechs would make more sense than the larger platforms, but at some point you just gotta enjoy the mecha