Decentralize exchanges are hard to use and they have low volume. Also I am not sure how you put your FIAT on them. Somebody obviously has to receive your FIAT and put it on the exchange and that's by no means "decentralized".
They are only hard to use if you have never used them before, in the same way reading the chart and placing limit orders on a centralized exchange is hard to do if you have never done it before. Once you've done a few peer to peer trades, it is very straightforward. Also, the fiat exchanges hands directly from buyer to seller, be that via cash, bank transfer, money order, or some electronic money service such as PayPal, Zelle, Advanced Cash, Perfect Money, etc. The fiat is never on the exchange.
Since all people in the world have fiat money and many want bitcoin, without centralized exchanges they'd have to meet up with people to trade. It would be almost impossible to do.
There is no need to physically meet up with another person to trade peer to peer unless you want to exchange cash in person. You can use any of the electronic methods I've mentioned above, and you can even send cash in the mail if you wish.
but if stablecoins will become much more popular, they will be easily regulated and start having the negative features of exchanges - KYC, frozen transactions, seized funds, and as they become more centralized, we'll start seeing hacks and inside jobs. You can't have a stablecoin without some company claiming to have $1 in their bank account for every token.
Many stablecoins can already be frozen or seized and have addresses "blacklisted", and there is also no way to prove that your stablecoins are actually backed up with the same amount of fiat. Indeed Tether, the most popular stablecoin, is known to
not be backed up as they claim, and is running a fractional reserve system. Stablecoins are worse than fiat in many respects.