Sure, they are not perfectly stable, there is always inflation (sometimes more, sometimes less).
But its not as volatile as crypto. In last 24h difference between monero high an low is ~2% (which isnt terrible by itself but on a longer period its larger difference) and regular currency effectively has almost no volatility in 24h period.
Acording to google 1 year inflation was ~3.4% for dollar. (so probably diff hig low is just a bit higher) Difference between monero 1 year low an high was greater than 20%. So monero is something like 6 times more volatile. (and i am not talking about inflation, but volatility)
So its not exactly the same situation regarding stability.
Why?