Because there’s something in it for everybody. The current global nuclear weapons-control regime operates on a dangerously untenable false premise: that only ‘responsible’ nations can or should be allowed to make and maintain arsenals of nuclear warheads. At first blush, it may seem highly rational: only those who will behave responsibly should have the most dangerous weapons; but, then, upon further examination, who is qualified to make that judgment?
Probably not one nation not specifically seeking to expand the “nuclear club” to include itself would entrust to an autonomous international body the adjudication of who is responsible enough to have the right to add more nuclear weapons to the global stockpile. Certainly, the US tends to oppose allowing any external body to judge its own level of inherent responsibility or sovereign rights. And international law, at present, forbids the proliferation of nuclear weapons.