Because they never had the alternative to fiat. Some professional players [1], tech workers [2], city workers [3], etc, are starting to get portions of their salaries in BTC. You can always take fiat from work and save it in BTC as well.

It won't be this volatile forever, as the market cap gets larger, the volatility goes down.

1: https://ftw.usatoday.com/lists/nfl-draft-trevor-lawrence-cry...

2: https://finance.yahoo.com/news/olaf-carlson-wee-first-coinba...

3: https://www.newsweek.com/miami-passes-resolution-allow-payin...