Such girls, and there are shocking many of them in Western countries of all races, are in general not held against their will, they have plenty of chances to leave, to move away from such a neighborhood, to change their life-style. However many of them enjoy to join as a gang-member and will hardly regret that until they are either sentenced to a long jail-term, in hospital with life-threatening injuries or even in a coffin because of drug abuse.
Maybe they cannot find any other girl in Western society? Young sincere girls and still available are rare, young men always widely outnumber women.I wonder why a guy would pine after a girl who wanted to be with a bad boy. It shows a lack in judgment. Why would a decent man want a 'bad boys' left over girlfriend like that?
Also a search for a sincere girl might be costly - as a young man without support from family members, especially parents and siblings, from where should this money come from? - I know best as I was also in such a position long time ago.
Another explanation might be - as you noticed - they are ' bad boys left over girlfriends' - which means they are at this moment available and as a man you need not to line up as her boyfriend no. 5 or 8.
These men are desperate, they see no other solution but decide to date single mothers, prison girls, drug abusers from the bar....
However these men, despite being honest in their life are lonely, I feel they are misguided - such women are not known to me to be grateful to ordinary men, unwilling to work hard for a better life and often during recovery period within a few weeks they soon stray away again into their former party life-style of sex, alcohol and drugs.
I was also lonely when young and failed totally to find any still available reasonable woman living next to me, so I was starting soon to look around in foreign countries.
Looking back I think this was a good decision I made. Look around outside of your own native country....