Taking gambling as source of income depends on individual and if you don't see it as a major source of income, so I can tell you that I have many friends that depends on gambling to make a living. It all depends on the place you come from if they don't take gambling as serious as you think. So many friends do not have a job of there own so they see gambling as a big survival slot for them to make a living and pat bills and is working for them.
Gambling should never become the major source of income, it's just not healthy. It can be a hobby or an entertainment factor, but depending solely on gambling as an only source of income is not that sustainable and I'm not sure how good it is for your mental health.
I know a few professional poker players that had made a proper career out of it, but I think that there's a difference between a poker player and a slots player.