From a use-case perspective, Haskell monads let you restrict where various effects happen. Some code can be allowed to do local mutation, other code can do shared-mutation suitable for (ACID-like) transactions. In Java you can do anything, everywhere (except throw checked exceptions from lambdas for some reason) so the monadic wrappers would give you about as much guarantee as a comment, and be less clear.

From a representational perspective, you need a generic of a generic to represent<M><A> and that doesn't really work. Or if you go for interfaces not type variables, you could have a Monad<A> but you don't know which one you have (List? Future? Parser? Either?). It's like representing the above as literal 'Object's, you'd be constantly casting.

because no matter how much java wants to be like scala, it's very painful to work with (verbose). There is no good Option type either. But I believe there are third party libraries out there that do a better job, if you really can't use scala for some reason