“I always wonder why those robots have to be humanoid.“

You are correct to wonder this and almost every use case for a robot will be optimized to a non-human form factor.

Certainly there are tasks - like BJJ training partner - that require a human form factor. Almost everything else, including general, purpose, helper, robot, will be cheaper and more extensible in a non-human form factor.

One of your children remarked that nature has experimented with form factors and humans have won… To which I would point out that the upright, bipedal, form factor arose from the limits of oxygen processing, and heat dissipation… Neither limitation will be encountered in the same way with a robot…

… or perhaps I would point out that nature has, indeed, experimented with form factors and ants won - by a very large margin.