Makes sense - but I'm still not clear on why it would need to be humanoid in form-factor or indeed self-contained/autonomous and not a more flexible, mobile, multipurpose robot-arm-like-thing?

Like is there any particular reason for it to be about 6' tall with exactly two 3' long, three-jointed arms rather than any of the other possible permutations for those things?

You want robots that can do tasks humans can do but robot arms can't - which has a way of driving the design decisions. The more you move towards "flexible, mobile, multipurpose", the more you move towards the humanoid frames.

Just look up how vehicle interior assembly is performed now. Look at all the things that are still done by humans - all the different assembly stations, all the loading and unloading, all the installation operations, all the panels and wiring harnesses and plugs and bolts.

Then try to come up with a robot frame that would do all of it - every single operation that's currently done by a human - while being significantly less complex than a humanoid frame.

The design space constraints would choke the life out of you.