For some people it's the same reason that people play games or watch movies - it's just fun. For others the community keeps them around. The inability to sit by and watch stuff be done the wrong way. The desire to improve process and the community when they can/want. A sense of empowerment not being met in their offline lives. Epeen.

These cover a lot of the reasons for why people would get into it. Many of these reasons are also why people stay but add to that the sunk time fallacy and FOMO.