Why is it that when a player wins and wants to withdraw their winning the casino will begin to ask for info like source of funds, KYC etc.
First of all, what casinos do you mean? Online casinos or physical casinos?
If you mean online casinos, not all casinos platforms do the same way. Some casinos may not really care about your source of funds, but for KYC it is normal to ask by them. Even centralized crypto exchanges, also require KYC for their members. The purpose is not only to secure the funds, but the KYC is a basic thing to have by common platforms nowadays. Anyway, if you don't pass KYC, you can move to other casinos which never require KYC, if any.
But they careless about asking for those information when the player is losing money in the platform.
What for? They won't send money to the losers.
Maybe, if you have big winnings, why not try withdrawing smaller funds at intermittent intervals?
Because if you do really withdraw all at once, the casino may really notice your account.
Most casinos have their ToS stating that they will require verification if it deems necessary.
In this case, it is their prerogative to ask the player if for them, your account suddenly withdraw large amount.
As a player, as much as possible don't make any reason that will make the casino suspicious about your account.