Commented-out code – like any other comment – ages badly. It needs maintaining if it is to remain in a state where it can be uncommented and compiled, should the need arise, and of course it won't be maintained in this way since the need is unlikely to arise. There is more than enough live code to maintain without devoting attention to half-dead code in comments.

Once code has been commented out it becomes hard to remove: someone at some point obviously thought the code worth leaving in, so future programmers working on the file honour that decision, although they may well consider the code smells a bit off – code which someone once found problems with, attempted to cure, never really got to the bottom of, and left for someone else to sort out, or, more than likely, ignore.