No `MonadFix ((,) a)`?

I feel like I must be missing something obvious but I couldn't find any information about this: why isn't there a MonadFix ((,) a) instance? There is one for Writer a in transformers, which seems lawful to me.

instance Monoid a => MonadFix ((,) a) where
  mfix f = let (a, b) = f b in (a, b)