People do join gangs for many, many reasons. Abuse and poverty tend to be most common, but by far not always. There are some people who join gangs in order to have some assistance on running their business. There are some people who join gangs just to have fun. And there are some gangs that are so embedded into the community that gang membership runs in families. Which again highlights the diversity of people who join gangs.
In addition, some people can be coerced into joining a gang when the gang is in a desperate situation, such as being theratened by a more powerful gang over territory or wanting to take over territory. There people are often coerced into joining by means of physical harrassment. Some of these people tend to stick, while others are dropped by the gang once the gang crisis is over.