They're primarily lending to institutional customers (hedge funds, HFT's). If you look on DeFi markets borrowing a couple billion dollars is going to cost you a

They're primarily lending to institutional customers (hedge funds, HFT's). If you look on DeFi markets borrowing a couple billion dollars is going to cost you a lot more than the 10% BlockFi is probably charging, due to how automated lending mechanisms work. The reason the yield is so high is because there's insufficient liquidity in ETH/BTC/stablecoins. Notice also that these loans are undercollateralized. DeFi lending requires overcollateralization which often is not appealing to HFT's executing low-risk arbitrage trades that make a few bps of profit. They need big leverage to make decent money. In our normal financial system banks are willing to lend them money, but in DeFi nobody does undercollateralized loans.