You cannot, but you can increase money supply money supply more stably when following average GDP growth, and increase money supply more when velocity decreased- and atrophy the supply when it increases. And a currency is much more than just what people can spend at a store. It is what people keep their savings in, what companies pay their employees in, what banks lend.
This cannot be done with an unstable currency- you cannot have a debt that will either go up or down 20% in value in the same year. I do not think fiats are inherently more stable, but some fiats have proven to be somewhat more stable because of responsible central banking- its not a good idea to count on central banks being responsible for ever. But essentially all widely spread cryptos continue to have a significant amount held by speculators and therefore they cannot be stable currencies.
The simple answer is that fiats are only more stable because their relative worth is more settled. For the same reason small stocks are unstable while big blue chip stocks are (relatively) not. If you look at logarithmic charts of any big crypto over time you can literally see the volatility tapering out as the market cap increases.
Of the cryptos that survived yeah thats been true, except for the speculative rushes encouraged by a lot of hodlers. Furthermore, even those transactions are high- the velocity is still much lower than in most fiats where people put their excess in a bank that then invests with it.
