It ebbs and flows. My personal conspiracy is that it's a built-in self-destruct switch in case a species overpowers all predators, diseases, and lack of resources. Some code auto-nerfs the species so they aren't OP forever.
When resources are plentiful, vaccines have stopped most major diseases, everyone is washing their hands and decently educated ..that's when the incels, the homeschooler mommy groups who distrust science, and the religious zealots sow discord and take civilization down, lol.
I'm sure the demographics throughout history change. But the base instincts of greed, fear, and hate blow apart cultures and empires throughout time.