American here, our gangs primarily exist because the police do not benefit (and actively harm) minority communities, and the gangs offer what is essentially an alternative police force* for crimes that otherwise go unenforced and unreported. Is that not the reason for gangs in Sweden?
(Gangs in these categories historically include the Italian mafia in the US, the Gangs of New York, the Triads, the Crips & Bloods, and I’m sure I’m missing others)