One issue with that is that you can write Flatmap in a way that doesn't obey the Monad axioms. And once you write out what it means to be 'correctly' flatmappable you've recreated the Monad axioms.

Though it would help if more people were aware that a 'nice' way to 'unnest' a functor (F F x -> F x) is really all that it takes to have a Monad.