Greed. It's called greed.

you don't know her, she goes to another cave

Neanderthals and early humans sitting in a tree, K-I-S-S-I-N-G.
First came love, then came marriage, then came the extinction of Neanderthals due to a combination of climate change, competition for resources, and advantages in technology and diet, as well as some degree of interbreeding

My god can we wake up from this nonsensical nightmare.