All of this! Misery loves company
Also, OP you’re asking why they mistreat their mate but you also need to ask why the mate stays with that toxic person.
Both are damaged/broken in some way to 1) treat people like shyt and 2) accept being treated like shyt. You can’t have one without the other
Yea when I was a kid one of the neighbors stayed whooping his wife. she never left. it was always something going on over there.