Why are there any negotiations at all then? If there are any kind of negotiations then you can do it better.

A primary goal of the Paris Agreement was signaling; the benefit of it was that they got all countries aboard, except Syria (which was in the middle of a civil war) and Nicaragua (which wanted more and voluntarily is doing more).

Trump's decision (assuming it's a rational one and not fed by ignorance) is signaling the reverse: he's telling his supporters that climate change is a myth. And this is why he's getting pushback from other countries.