Right, that's a bunch of countries agreeing not to develop nuclear weapons, but it doesn't include all countries. So what's the point? A few countries deciding to flout the treaty and develop their own WMDs means they can just threaten everyone else who followed the treaty. This is exactly what North Korea is doing. Now their neighbors are considering abandoning the treaty and joining the nuclear weapons club.
A treaty like this is basically worthless if you can't enforce it.
The point is managing risk. The more nuclear powers there are, the more likely is that something goes wrong. And so far we don't have countries leaving NPT in the dozens so I'd say that it is working rather well. Not perfect, but well.
A treaty like this is basically worthless if you can't enforce it.