I just don't understand

I really don't understand how girls can date somebody that doesn't take care of their kids. My ex is 18 (I'm the same age as him) his new gf is 15, they've been together for a few months and she isn't even a little bothered that he doesn't take care of his child. Like how do you do that, I couldn't date somebody like that, ESPECIALLY if they were out here spending money on me but not their own child. It's just fucked up. I mean the main reason we're not even together anymore is because he wouldn't help with his son.😳