The city has a police force
Maybe success in life requires a few shortcuts and bending the rules. White collar crime may put a few extras in their pocket.
Blue collar crime is also rife in the city. Do they want to peddle something illegal? Or maybe rob and intimidate!
If they are serious about crime as a career, they will probably need friends and so joining a gang will get that sorted.
But beware, the city has a police force who will be looking out for the criminals among us. It could be a career choice for you to join them?