They are, they just aren't buying them back on coinbase. Tether don't manage the secondary market (cexes and dexes)

Though the minimum redeemable amount is $100k https://tether.to/en/fees

Why they are depegging is people are selling for under a dollar on coinbase (perhaps in panic); and Alameda Research (re: FTX exchange) who normally makes a ton of money doing the arbitrage between dollars and tether is having liquidity problems, so not doing it currrently.