It will be that way till more exchanges add currency pairs between altcoins and fiat. Currently altcoins all drop along with BTC, since BTC is paired with all altcoins and very few of the altcoins are paired with fiat, except on certain niche exchanges.

Also, while many might say that most altcoins are paired with USDT, I'm not sure I would count USDT as fiat.

Should be interesting to see what happens when Robinhood opens up trading for the handful of altcoins it has listed on the app at the moment. If I'm not mistaken, they would be the first to pair that many coins with USD.