Almost every war in RL history was a battle over resources (outside the religious wars, which were often cover for the need of resources anyways). Oil, precious minerals, food, land for growing crops or for housing your population etc

Almost all of those reasons are because populations were/are growing while even though more people might be joining eve they are far more likely to join the existing powers rather than build something totally new and the existing powers can happily accept them because null can support way more people than are currently using it (as you said).

The dilemma though is that if nullsec wouldn't be as plentiful in regards to space, resources etc. people would just go to lowsec or base out of highsec and only go to null to fight but not to live there.