Looking at these 2 graphics side by side; if I peg something to the above currency and call it 'stable', I should also be able to peg something to the below currency and call it 'stable', right?
The difference is the time frame: the dollar is quite stable from today to tomorrow, while Bitcoin can easily lose or gain 20%. But I've also seen the dollar (or euro) gain or drop much more than 1% in a day, after some announcement from a central bank.
Sure, the time frame is different. But as you say, fiat currencies
can also fluctuate quickly. Let's keep in mind that there are also other 'fiat stablecoins', so even a coin following the
Mexican peso would be called stablecoin. Even though it went from $0.70USD to $0.05USD in the last 10 years. Relative to the Mexican economy and to Mexican Peso holders, such a stablecoin would indeed appear 'stable' and not have lost any value.
I remember some weeks in 2018, where the TRY was inflating so hard that it 'out-crashed' Bitcoin, so measured in TRY people would have made profit holding Bitcoin (even though it was losing tons of value when measured in USD) compared to a Turkish Lira stablecoin. In that sense, Bitcoin was more stable than a theoretical stablecoin.
Now that I think about it, it makes me wonder if Bitcoin is called volatile to distract from the continuous drop in value of fiat money. Fact is: there is no stable currency anymore.
I don't believe in conspiracies hiding behind every corner, but I do believe that the majority of people sees their country's currency as stable and everything that loses or gains value, when measured in their local currency, unstable. This is a general thing that I also noticed before Bitcoin and / or in other contexts than Bitcoin.