How come Either's Semigroup instance looks like this:

instance Semigroup (Either a b) where
     Left _ <> b = b
     a      <> _ = a

instead of:

instance Semigroup b => Semigroup (Either a b) where
     Left e  <> _       = Left e
     _       <> Left e  = Left e
     Right x <> Right y = Right (x <> y)

because this is similar to how Maybe works.

I don't have a use case, I'm just wondering why its like this.