... but can't they skip or ride bikes?
I think they avoid skipping because body parts would fall off. I also suspect a lacky of muscular coordination, or even muscles, may contribute to being less lively.
As to not riding bikes, I think it's just tradition. AFAICT, there's no tradition against riding horses. For example refer to the "Night on Bald Mountain" sequence of Disney's "Fantasia". It definitely portrays a
deadster decay-challenged dead person riding a skeletal horse. It's probably BYOB (Bring Your Own Bridle), so don't forget to leave clearly written instructions about your grave goods.
Ancient Egyptian tomb paintings also show the dead on chariots, on boats (yay! boating!), as well as walking. So give it another couple of millenia, and maybe bikes will become acceptable. When you're dead, it's just a question of time.