Most do - that's exactly what they agree to including in international law by signing the non-proliferation treaty, that the five existing nuclear powers stay that way and everyone agrees to work to stop any more countries getting them (in exchange for access to uranium for fuel, information, equipment etc). A few countries aren't signatories (notably India, Israel and Pakistan) and North Korea withdrew from it in 2003; Iran, however, signed it and remains bound today. It's not just the existing nuclear powers, it's almost everyone who agreed that nuclear proliferation is a bad thing.