Its a job that gets you middle class wages without going to college (in many large jurisdictions even) or the military and a retirement with pension by what, 50, at which point you move from county/local cop to state investigator and start working towards your second retirement. Oh, and you can work side jobs as an off duty security guard. It can be an extremely lucrative career if you stay out of trouble and move up through the ranks. If you really have aptitude and pick up a degree along the way, you might get picked up by the feds and then ride that out until 55 and then back to a state investigator job/managerial job for a sweet sweet paycheck.
Oh, side jobs-some are working ball games or concerts or things like that. Don't forget the cheap food at restaurants!
So, no, they don't go into this to get rich, but it is a seriously good way to make a living if you can keep your nose clean and move up the ranks. Much better earning potential than a lot of jobs that require degrees.