Hey ! I am not a robotics expert, but I work at this company (Wandercraft) as software dev, so I know some of the reasons :) First of all I'd say that's because it simpler: we have so much to do that simplicity is a strong decision factor. That way you also reduce the number of moving parts, so maintenance is easier. On the software side you also get the marginal benefit that you keep a 1:1 mapping between joints and actuator (an example with coupled actuators: Cassie, from the Oregon State University). That way you can have different control strategies for the different joints.