I know, it's weird, but there are three reasons for this:
•the pokedex has very unreliable information. For instance, Yanmega can carry a full grown adult, yet it can't learn Fly.
•it says when it's belly is full it can't move a finger, maybe it's belly is not full when using Metronome.
•Gamefreak made it as a joke, like splash.
There may be more reasons, but these ones make the most sense