"If the current human population were reduced by 98% there's basically nothing to fight over."
Human population was a fraction of its current level several thousand years ago and there was a lot of fighting between tribes, not to mention inter-tribal violence from the strong towards the weak.
"Loads of land, food and water for everyone."
Finding enough food to keep going took up most of everyone's time. If one group of people had easy access to clean water, a stronger group came and pushed them away. Strong preyed on the weak. People were enslaved. It wasn't all pastoral heaven, as people taste the abundant fruits of Eden.
"That said, I wouldnt describe SE as Sci Fi. I'd describe it as post-apocalyptic accessible literary."
Post-apocalyptic is a sub-genre of SF. It is "accessible", for sure, but that is because it is not terribly clever. It's chick-litty SF.
Nothing wrong with enjoying a light read, but it really shouldn't be the best SF anyone has ever read. There really are many excellent books that explore a post-apocalyptic world.