The lending rates are higher because lenders are paid a premium for owning a potentially worthless (USDT denominated) credit. The idiom that comes to mind is "picking up pennies in front of a steamroller".
https://compound.finance/markets/USDC
https://compound.finance/markets/USDT
For example, on October 7, it shows USDT at over (sorry, "north of") 13%, while USDC was (sorry, "clocked in at") ~5%.
The demand for Tether comes mostly from Binance and the other centralized Asian exchanges. The demand for Circle dollars comes from the DeFi ecosystem and FTX.
The lending rates are determined by a separate mechanism and reflect the probability of a future departure from the peg.
In the depths of a crash, the potential is definitely there. Last time Bitfinex bailed out Teather when its reserves came up short. So to really see Teather fail, we probably need a crypto crash and a weakened Bitfinex.
This is why people still use Teather. Even though it is risky, the current market dynamics make a failure unlikely.
IMHO failure due to embezzlement (Teather reserves stolen by an inside man) is a real possibility. The management team has already proven to be very shady...
How much crypto is borrowed on platforms like Compound against USDT? That collateral takes a 5% haircut, we’ll see painful liquidations. 10% or 20%? Fire sale everywhere.
Close to zero. Compound, Aave, Maker - none of them allow folks to use USDT as collateral to borrow against. You can lend it for yield, and borrow it from others, but you can't use it as collateral.
There are some riskier protocol that allow it (rari, cream) but those are much smaller.
I would speculate that the premium is because of USDTs network effects -- the USDT/crypto trading pairs are the most liquid on centralized exchanges.
There is no reason that holding any "worthless" commodity by itself should incur any premium on the market.