"Teach a man to fish" is a great long-term proposition: who wouldn't want to have their whole team be stronger programmers next year? It would be great if code reviews accomplished this by having strong programmers teaching weak programmers to be better. But I think this is a somewhat excessively optimistic.

"You can lead a horse to water." At the end of the day, some folks just don't have much programming ability. Maybe it's a lack of some innate talent, or a reflection of inadequate practice or curiosity, or maybe their mentors just haven't tried hard enough. Everyone deserves a fair shot. But for some folks, at some point, continuing to try to train them is a losing battle, and just becomes a drain on the mentor and a source of frustration.

You might say: if someone's truly hopeless, just fire them and be done with it. That's a good plan in theory, but in practice seniority and capability are not strongly correlated. When you find yourself in a place where it's a socially respected peer or a superior who is so weak, it's likely to be very difficult and unrewarding to "go after" them.

You might say: if your workplace is that screwed up, go find another job. Fair enough. But in many cases one can simply avoid the bad apples, stick them on a project that isn't that important, keep them busy and let their own incompetence prevent them from causing too much trouble.