I swear I've read an article on exactly why human-scale robot servants make no sense.
It's something like:
1. Anything human-scale will tend to weigh as much as a human. That means it needs a lot of batteries, compared to e.g. a roomba. Lots more material and lots more weight means lots more cost. 2. Also, they'll be heavy. Which means if they e.g. fall down the stairs, they could easily kill someone. 3. If they run out of power unexpectedly (e.g. someone blocks their path to the charger) then they'll be a huge pain in the ass to move, because they're human scale. Even moreso if they're on the stairs for some reason.