Why would a girl like a objective "loser"?

This girl that I like. She also has a crush on me and we flirt a lot is with a dude I could only describe as a typical loser. He is 24 lives with his parents. Has a Dui. Is fat and unhealthy. Is a nice guy that gets disrespected by his girlfriend. She is always insulting him and they fight often. They have also broken up multiple times. She crosses relationship boundaries often. She sat on my lap which would be a big no no if I was her boyfriend. but he doesn't care cause he's a pushover. My question is why is she still attached to this guy. She has talked about how she can't be with him anymore. They have been dating for 3 years so I understand that she's invested in him but still. I don't understand why she doesn't go with me. I have all my shit together and im only 20. Im sure there relationship will collapse sometime soon but I dont understand why she is still hesitant. Any insights?
Why would a girl like a objective "loser"?
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