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.
It can be but as Charles-Tim has commented not even 5% of sports bettors are profitable long term. For those who are profitable, it is a source of income, which can be exclusive if they dedicate themselves solely to it, or an extra income, which is more common, if they have a job and sports betting is an addition, or they are professional sports bettors who have other streams of income such as YouTube channels, coaching, or others.