It isn’t though. A monad is just an abstract mathematical object that satisfies a few axioms. It doesn’t have anything to do with implementations or computations, let alone objects in the OOP sense.
It’s better to think of it like this: a value in a box is one example of a data type that can satisfy the monad axioms. Another example is a delayed computation that produces a value when executed. Yet another is a delayed computation which produces no value at all but causes a message to be printed to the screen.
The problem is, of course, that a monad is not a value in a box. That's a misunderstanding, resulting from an oversimplified metaphor. This shows a frequent problem with metaphors.
It’s better to think of it like this: a value in a box is one example of a data type that can satisfy the monad axioms. Another example is a delayed computation that produces a value when executed. Yet another is a delayed computation which produces no value at all but causes a message to be printed to the screen.