For a useful robot arm, you need high forces/torques. Usually, this is achieved by high ratio [1] gearboxes between the motor and the joint. Those are expensive, and inefficient and make estimating the output force via the motor current almost impossible.

[1] In the order of 1:100, see e.g. https://www.harmonicdrive.net/

There are arms using steppers and 3d printed epicyclic or cycloidal drives.