Maybe many of them are just butthurt like what you said but also there are some that have been cheated by gambling sites as we also knew that not all gambling sites operates as how we are seeing and knowing them now because there are other side of each gambling site.
that I also believe that many of them will try to take players money but when they are being called then things changed and indeed they are paying the taken money from those gamblers sad but reality , while other sites pretend to be not intended to act such but deep inside its their tactics to take fund from those who have no capacity to run against their behavior .
You are right but the odds of reputable casino platforms doing something like that are very low and we know that most casino platforms we have in this forum are at least somewhat reputable apart from the ones that are being announced newly, and if a gambler chooses a new casino and faces a situation like that, we can't blame them because the platform is new and doesn't have a high reputation in the community yet but this is also important for gamblers to make sure they are not depositing large amounts in fairly new casinos.
Nothing can be done about those who are butthurt because they have lost their money in a casino and now want to accuse them of being scammers. Some such gamblers would even create fake proofs such as fabricated or edited chat screenshots that they claim are real and were done with the customer support team.