While this might be the case for many of us, there is definitely a group of people who like risk and uncertainty. If you don't look for it you go for a safe 9 to 5 job without any upside. Your salary is fixed and you won't make any big pay jumps. People in the finance industry work a lot based on bonuses, so their salary is uncertain too. Or people working in sales are paid by commissions. There is also a change your salary per month.
Although both gamblers and employees get uncertain income, at least employees don't have the risk of losing money or the worst possibility is that what they did doesn't get rewarded, so they will only experience a loss in time and energy. Whereas in gambling, apart from an uncertain income, every gambler will definitely face the risk of losing their money.
Pro gamblers get rich not because they win a lot, because when against other pro-gamblers, their chances of winning even out with each other. Now you might be asking why or how do they earn the big racks, which is of course due to publicity. More often than not they will advertise on the themselves pretty much as superstars which would then allow them to gain projects which offer more consistent pay and security. They may win, but that is not solely where their money comes from.