Because affiliated businesses (Bitfinex, Blockstream, FTX, Celsius, Binance, others) that hold btc as collateral against tether given to them are protecting their interests. Alex Mashinsky of Celsius has stated that if btc price dropped, they were required to purchase more btc to rebalance. This would be like a bank expecting property owners to purchase more property if prices dipped and the bank even lending you more to do so. Artificially creating demand did not end well in 2008 with subprime mortgage crisis.

EDIT: Big exchanges have close relationships with tether, which facilitates liquidation of shorts/longs and front running. Any exchange needing liquidity has easy access to tether if they ‘co-operate’.