Because they are still immature at some level. That said, many guys are mature and awesome human beings on other levels of life and kudos to them for that, but not when it comes to relationships. And they really need to take a good hard look at themselves if they want that to improve. It always surprised me how a person could put so much energy into a career or a hobby but just 'expect' a relationship to magically manifest and maintain itself. It takes enormous commitment.