Because that’s pretty much the definition of nation-state: control over a territory and what happens inside it. If you meant to ask why they should belong to the nation states, well, if they belonged to “humanity” someone would still be calling the shots, and there’s no guarantee it would be the Norwegians.

If we abolish nations and go for absolute globalism, who should extract the oil ?

Who should decide who gets to extract the oil ?

If there's a system to solve those two questions, why is that system better(as you imply it would be) than the current system ?

Here's a reason: some small populations like Norway and New Zealand are successfully going greener. This happens in part because our politicians are paying attention to their constituency.

It appears to me that many larger countries are not going green (in particular the US climate change denial) and part of the problem is that those at the "top" are very detached from those at the bottom.

Why would a single global political entity care more for the median person or the poor person? Care more for a forest, or a global oil company?