To hide their identity. This is one of the major reasons why people ask others to verify an account using their name.
But do you know it's even riskier using someone else KYC details for your trading/gambling account? Because what if you have a pending transaction that requires facial verification for confirmation, and the person whose details and face you used is either died or has traveled. What will you do at that moment? Because to me, its better to apply using your details or equally avoid such casinos to prevent future unforeseen occurrences, just like the one I experience a few days ago, sending $100 worth of BTC to my Binance account, just for me to try to sell on p2p and I found out that my fund was locked due to the fact that I haven't uploaded proof of my residential address, which I did and a few minutes later fun was released. So imagine if I had used someone else details for my KYC, what do you think would have happened?