Yes it's true. Gamble is not what a gambler will take as a side job because it's not a thing that will provide a constant inflow of money. Many times luck is not always there for us when we are gambling, it comes ones a a while. Money one should gamble with is a type that one can risk then also afford to lose at ones. There are games when we can always think that will provide winning for us but most time we don't win at all
If most of the time we lose, then why do you think that gambling can be considered a side job?
In my opinion it is entertainment, and as we know for any entertainment the client has to pay. Whether it is a movie, dinner at a restaurant, skydiving or gambling. Whether you win or not depends on the reward algorithms, but this algorithm will not reward you too often, because the casino always has an advantage over you and someone has to pay for the operating costs of the casino, buying a license, etc. Gambling is far from a side job.
of course it's the minor part of all the gamblers but some people can make money over the long term only by gambling...
though I agree that it's not the best style of side job...