I think it's more like the non-keyworded default definitions of class methods, for the same reasons; the default definition has to potentially be valid for all instances of the class.

It's the difference between

class Applicative m => Monad m where

return :: a -> m a

return = pure -- always valid, but can be overridden in instance declarations

and

class Fuctor f => Applicative f where

(<*>) :: f (a -> b) -> f a -> f b

default (<*>) :: Monad f => f (a -> b) -> f a -> f b

(<*>) = ap -- only valid if matches the type signature above