Assume every motor has a 1% failure rate per year.
A boring wheeled roomba has 3 motors. That's a 2.9% failure rate per year, and 8.6% failures over 3 years.
Assume a humanoid robot has 43 motors. That gives you a 35% failure rate per year, and 73% over 3 years. That ain't good.
And not only is the humanoid robot less reliable, it's also 14.3x the price - because it's got 14.3x as many motors in it.
[1] And bearings and encoders and gearboxes and control boards and stuff... but they're largely proportional to the number of motors.
For example, do the motors in hard drives fail anywhere close to 1% a year in the first ~5 years? Backblaze data gives a total drive failure rate around 1% and I imagine most of those are not due to failure of motors.
But the neat thing about my argument is it holds true regardless of the underlying failure rate!
So long as your per-motor annual failure rate is >0, 43x it will be bigger than 3x it.
43x of 1% failure rate is tragic, but 43x of 0.1% is acceptable in my book.