I would think the key reason would be the NPT’s effects on other countries. All signing countries are forbidden from trading nuclear materials, technology and research linked with nuclear weapons AND power to countries that haven’t signed or are in violation of the treaty.
I think as long as Israel keeps up the lie they are allowed to import nuclear fuel for their domestic (civilian) reactors and participate in international conferences ect. And most powers are willing to allow that.
So in effect denying they have nuclear programme enables them to keep the programme going.