[XorNot]

The third arm would put a load in one direction of the robot. You can no longer balance by swinging 1 arm back, you have to swing both of them, or involve a leg motion you might not want to because the third arm has a more limited range of motion.

Hands are just hard: no one is building good hands yet because the materials science and motors isn't there to do it (see the production Atlas's with the 3 finger grippers).

And of course training data: we have a wealth of examples of how to move a bipedal platform around, but there are no 4-armed humans so you'd be figuring out the balancing from scratch (but it also has the same problem: 4 arms in physics terms is still basically 2: if you lose your balance you'll need to involve both arms to correct it).