Why choose to master A and not B?

Why do babies try to master a foot pedal-and-mobile combination?

For the same reason people climb mountains: because it's there.

IOW, opportunity and interest. (One might reasonably say there are people who climb mountains for status, but it's hard to pin a status motive on babies.)

In my observation, there are at least four major categories of reward mechanism: what I refer to as status, affiliation, safety, and stimulation, or S.A.S.S. for short. These likely correspond to major chemical pathways - maybe serotonin, oxytocin, ?, and dopamine.

Trying to make everything about human motivation into a single drive (status) makes it really hard to actually predict behavior, in my experience, since there are plenty of examples of people lowering their status to get one or more of the other kinds of reward.