More than 50% of the world population gamble in one sport or the other and some are making good money while some are still hoping and praying for their luck to shine.
I don't think we have up to 50% of the world's population as gamblers, I don't know how you arrive at such conclusion, maybe you can add a link to any research done in such area.
Gambling can be a source of income depending on which side of it you scale you fall into; if you own a casino or a shop, then you are have a guarantee that gambling can be a source of income. But if you are just a regular player, then there is no guarantee that gambling will be a source of income for you.