Most of European countries could have had nukes by now if they weren't stopped by the US/USSR; going by your logic it was inevitable once the UK and France had them the others would follow but they didn't. Of course at the time at least the American leadership was a bit more (forward) thinking than right now.
If you are the only person in the room with a gun, you have a huge advantage. With each additional person getting a gun too the advantage will be less, but it will still make sense to try to stop that process until everyone has a gun, and we are very far from that point. It is actually cheaper to try to stop proliferation than to build your defense with 'everyone has nukes now' in mind.
Most of European countries could have had nukes by now if they weren't stopped by the US/USSR; going by your logic it was inevitable once the UK and France had them the others would follow but they didn't. Of course at the time at least the American leadership was a bit more (forward) thinking than right now.
If you are the only person in the room with a gun, you have a huge advantage. With each additional person getting a gun too the advantage will be less, but it will still make sense to try to stop that process until everyone has a gun, and we are very far from that point. It is actually cheaper to try to stop proliferation than to build your defense with 'everyone has nukes now' in mind.
It's a failure of longterm thinking.