Not all casinos are like that, the ones that have a great reputation don't just attract players and make crazy promises. They're truthful to their rules and words and that's why they're being swarmed by customers for being like that.
Aside from that, everybody likes promos and that's another reason for customers to flocked in.
Indeed, the casinos with good reputation don't make crazy promises, but still have good enough promotions and that combination does the job.
About promos... there's an ugly catch (or more).
[1] Free money tends to need gambled more before getting withdrawn (and some don't read the rules and blame the casinos).
[2] Free money also attracts scammers who try to trick this and that in order to withdraw that free money (and when caught and banned they blame the casino).
For a new casino it's sometimes hard to tell whether they are in trouble (or scamming) or just watch their back and those claiming they've got scammed are from categories [1] or [2].
This being said, reputation is hard to earn and maintain, but a player/customer has to learn how to "read" all the calls of being scammed.
And of course, there's KYC. Some will display KYC requirements and will only ask for that in case of big money or something fishy. Others will hide behind KYC requirements in order to simply don't pay the winnings, where possible. KYC is in theory required by the law. In reality, since all stays in the crypto world, imho the KYC requirements are not needed, or, as yahoo said, they're bullshit.