You also should remember that big wins like that are not that commonly withdrawn. Most gamblers end up spending more than they win, so they will likely be spending that money rapidly more than they withdraw. Meaning casinos having to verify big wins - that frequency drops. I am sure they get wins to verify everyday but they will see that person wadte it more commonly.
They are running a business so obviously to maintain their name they will make sure to clear these things quickly.
Why people are so interested in casinos? Because they give big wins. Although not everyone can win, those who are lucky have won big wins. If casinos did not give big wins, then there would not have been so much attraction towards casinos among gamblers and so much trust would not have been born. I have not heard anyone complain that any casino platform could not withdraw big winnings if they used a good platform.
I will completely agree with you that in case of big wins, most of the gamblers later lose that money. They spend that money in the casino itself, which is why they fail to withdraw big money. Casinos are always in profit, so giving money to lucky people is not a cause for loss. Rather, the news of big money withdrawals attracts ordinary people to casinos even more.