because first of all people who aren't living in their parent's house have to pay for stuff from groceries to rent and utilities. and to pay for all that they have to have fiat because the grocery store, the bank, ... don't accept cryptocurrencies.
secondly what you refer to as "stable currencies" are just altcoins like any other, with the difference that these are 100% centralized. and in case of the most popular one tether it is the shadiest and most used at the same time.
assuming someone was naive enough to do this and the next day bitfinex was shut down by the FBI their lives would be ruined in a blinking of an eye because they lose all their life savings in a centralized shitcoin with no guarantee.
well the solution is go in crypto
the usd and eur most common crypto stable currenies out there.