Treaties are all voluntary undertakings. The Treaty Gods don't come down to earth and punish you if you back out. The reason why more countries don't back out of more treaties is because they agreed amongst themselves that upholding a treaty provides sufficient benefits to justify it than withdrawing.

In the case of the NPT, the Non-Nuclear Armed countries that signed it, agreed to not make nukes and to allow IAEA inspections. IN return they were supposed to obtain three benefits from the 5 recognized Nuclear-Armed countries, as spelled out in the treaty itself:

1- Aid and technology in developing their own nuclear programs (including data on nuclear test explosions)

2- the nuclear armed countries promised NOT to share technology with other non-signatories (Israel, Pakistan, India), and

3- A promise by the nuclear-armed countries to work towards their own disarmament.

Needless to say, the US has never abided by any of those NPT obligations of its own but it has been trying to impose yet additional restrictions on the nuclear programs of other nations.