Considering these are backed by companies most of the time, that means these companies would have to merge together as well and I doubt that would happen, not just economically but legally neither. I mean lets take a look at USDT which is the biggest right now, Binance has BUSD, so Binance would have to swallow bitfinex and tether as a whole, which I doubt legally could be done, even if they can afford to do that, which they probably can't. This is the main reason, its backed by companies and as long as they are, it will not happen. We need to stop using the major ones though, it gives them so much money and power, the centralization is horrible. The day tether gets banned everywhere, will be the day we realize our money is not backed 1 to 1 ratio and they can't pay us all.
This answer why we don't really need a stable coin at first place. Any currency will reach a certain stability by time goes by like with FIAT money. Although we call it stable coin refering to the USD while the dollar itself is not that stable.
This form of tokens was mainly created as a trade instrument within exchanges.