Once there is a huge money involve you better expect a KYC, not just in casino but for every other places too where there is a money involve especially in banks. And also, you can read every ToS on different casinos if you don't know it's suppose to be mean. It will benefit to both gamblers and the exchanges for security reasons and also to avoid money laundering.
Even if you say that a player is just playing but once they hit the jackpot, then that's it, KYC is served.
this is part of keeping their gaming license. the AML policy is enforced so they also have to abide such policy. and one way is to require KYC from their clients to keep that license. they also have no choice but to follow such protocol. as a player, it is always in your best interest to read the terms of the site. if they have gaming license, expect that you may need to submit kyc at any point in your gambling experience from them, especially if you are a high roller.
in some cases, this is one way to stall or freeze big withdrawal from a player, if the casino is running shady.