Two things. First, the humanoid is the result of millions of years of recent evolution. It's a solid design. Sure, you probably can come up with a better design, but why throw away what already works? That's wasteful. Second, we have millennia of human technology designed for the humanoid form. Why throw that away either? Same argument about waste applies.
Third, you don't want to waste the opportunity to make your robot look like frickin' Boba Fett. Same argument about waste applies yet again.
I have the simplest tastes. I am always satisfied with the best. -- Oscar Wilde