Why discarded values are () instead of ⊥ in Haskell?

Howcome in Haskell, when there is a value that would be discarded, () is used instead of ⊥?

Examples (can't really think of anything other than IO actions at the moment):

mapM_ :: (Monad m) => (a -> m b) -> [a] -> m ()
foldM_ :: (Monad m) => (a -> b -> m a) -> a -> [b] -> m ()
writeFile :: FilePath -> String -> IO ()

Under strict evaluation, this makes perfect sense, but in Haskell, it only makes the domain bigger.

Perhaps there are "unused parameter" functions d -> a which are strict on d (where d is an unconstrained type parameter and does not appear free in a)? Ex: seq, const' x y = yseqx.