I've been on Mturk for about a year and a half with 7700 approved HITs, mostly surveys. Many of these surveys say the end a random worker will be chosen from a raffle to receive a prize. Usually $20 to $50 or gift cards. I never count on winning these or hear back from the Requestors, but I would think that with as many I have done I would have probably won at least one. I personally don't believe these raffles exist, but I curious to see if anyone has ever won.
It's a raffle. Just like any lottery, chances of winning are pretty low so don't think too much of that or expect anything. And yes, people do win. I've seen screenshots of bonuses with the winnings, and I won a $25 Wayfair gift card before.
Not that there is a way to check that they're actually doing it, but chances are there is a real raffle for most of them. Have seen a handful of winners through the years but I never have personally won. Most hits have a minimum of 200+ participants, some with thousands so the chances are still slim