Reasons:
- To control spam
- To avoid users who uses multiple accounts
- To avoid cheaters
- To avoid abusers
- To get personal infos and used to illegal works (It might be right?)
KYC doesn't help to avoid multiple accounts and cheaters, I know some people that pass KYC for 30 -50 accounts in good projects and then sell these accounts to investors who want to invest but don't have an account, in similar way a bounty hunter can pass KYC multiple times using documents of his/her friends or relatives. How does KYC control spam? It's unclear for me.
Actually you have a point, but that is the reason why most of the bounty campaign are needs an KYC for bounty participants to avoid multiple accounts or cheaters.