Any large winnings are of interest not only to casinos, but also to tax authorities and regulators. Therefore, it is not surprising that once a person wins a large sum, the casino requires him to pass KYC and provide various documents.
If you play casino with a licence and a regulated one you must be ready to pass KYC in all the cases. The requirements of regulators become more strick and it is obligatory to pass KYC neally in all the jurisdictions.
Now there are a lot of regulated casinos and users have a big choice of casinos to play. For every casino liquidity is important. A lot depends how many users the casino has. That is why every casino is interested to have a lot of users and it must to interested them to stay with casino for a long time.