People cash out to stablecoins when they don't actually want to exit completely. If you hold stablecoins, that can be to sit out a bear market you expect to tank the price, or to exploit arbitrage opportunities between exchanges, hence the reason Tether decided to issue USDT on faster networks like Ethereum, EOS, TRON.

Moving actual fiat between exchanges is very time consuming and burdensome, while moving stablecoins between exchanges takes a few minutes at most through aforementioned networks.

People may not like stablecoins, but they prop up the price of every single crypto. It's capital that doesn't leave the market as quickly and regularly as actual fiat does, especially during bear markets.