[blam]

"I don’t really understand how Neanterdals aren’t considered human, but we can breed with them."

They are considered human. There were many different humans that are now extinct.

The Humans Who Went Extinct: Why Neanderthals Died Out and We Survived

[Jonty30]

I am familiar that one theory states that Neantherdals needed for more energy to do the same job and that because they only hunted the biggest game, which required extensive travel, they were also less efficient at reproducing themselves.