There was a period during 2016 to 2018 when many of the crypto casinos were very new, they had low reputation and their admins were anonymous. A random "newbie/brand new" user could open a topic in BitcoinTalk and make an accusation, and because many users in the forum post/bump the topic, it would make the accusations sort of believable because the topic sort of had some acknowledgement in the forum.
¯\_(ツ)_/¯
Today, it's simply harder to do.
Yeah. This sort of shit will never stop, but it's a lot easier to differentiate between the legit and fake accusations these days as you mentioned yourself. This is why they dropped to a certain extent over time.
One of the easiest ways to separate them effectively is by checking whether they are willing to provide proper proof or not.