Can you imagine stepping on a rock when every bone in your foot/ankle is fused?

Seems like a recipe for broken bones and never being able to run anywhere, ever.

Edit: I shouldn't need to explain why the bones in your hands aren't fused, assuming this initial comment isn't a joke in the first place.

Those are the ones we move the most.

Because then they would be immobile without any joints between the bones. If they were one big bone, they’d essentially be a block of wood shaped like a hand/foot