As an example, even the most anti-immigrant countries refused to leave the UN convention on refugees and Canada was reluctant to leave the UN Convention on illicit drugs.
But what's the big deal about UN conventions? Why treat them so seriously if none of them are enforceable anyway?
When a treaty isn't enforceable or has no real consequences of being non-compliant its simply better to stay in it, unless the goal is to make a statement by leaving. Announcing intent to leave a treaty sends a message that you aren't willing to cooperate on whatever the treaty was about. Leaving also generates a bigger news story than simply being non-compliant, when a country is non-compliant with a treaty they can downplay the negative press. A country can disagree with the scope of what illicit drugs are, but still be committed to preventing the spread and abuse of those illicit drugs they do agree with.
Leaving a treaty about refugees sends the message you aren't interested in helping any refugees ever and not willing to negotiate about it at all. Being a non compliant member of the treaty leaves room for negotiation, you can support aiding refugees but object to the scope of that aid and still be helpful to a degree.
The UN was created to create an environment of international cooperation and harmony to prevent future world wars, genocides, and promote development of non-industrialized nations. It was also created in such a way that it can't really interfere too much with the actions of superpower nations. In order to achieve these goals there needs to be a sort of collective agreement/delusion that the UN is really more powerful than it actually is. Countries outright leaving treaties is a fairly major blow to maintaining that illusion and the stated goals of the UN, while quietly being non-compliant keeps everyone happy.
