Because for some people who do not agree with the KYC request they feel disturbed or find it troublesome.
Yes, in fact those are one of the things that are always handled in Casinos, the issue of kyc that nobody likes, but what you say is very true, if you are an average player, who has decent earnings that are not so high, because the kyc is not so radical, but for the whales they do ask for much more documents and it is something normal, since you bet with more money they agree to the Casino's demands because they require everything they can provide, I imagine this too which is for the protection of the players themselves, although as I said before, I am not very given to the kyc, nor for the average player or whale.
Problem with other average gambler or small time gamblers they are over exaggerated on things which is manageable with. Besides if they are playing on reputable casino then there's really no problem with this, maybe we can only got an issue if a new casino will ask this requirements and hold our withdrawal for questionable reason then maybe for that we might gonna face more trouble with them.
One of the minor issues that is usually raised is about KYC at new casinos which often withholds customer funds and it can be resolved it only takes time but impatient and angry customers raise the issue and what's worse they sometimes make crap about the casino the.