Dozens of joints, one coordinated body
A typical modern humanoid has somewhere between 20 and 40+ degrees of freedom: shoulders, elbows, wrists, and fingers on each arm, a torso that can bend and twist, hips, knees, and ankles on each leg. Every one of those joints needs its own actuator (motor plus gearing, or increasingly a hydraulic or electro-hydraulic unit), and all of them have to move in a coordinated way to produce a single useful motion like picking up a box or climbing a step.
This is a fundamentally different engineering problem than a six-axis industrial arm bolted to a factory floor. That arm only has to coordinate its own joints against a fixed base. A humanoid has to coordinate all of its joints against a base, its own feet, that is itself moving and potentially unstable. Every arm motion shifts the robot's center of mass and can threaten its balance.