First, why would they use stablecoins when they can use cashless transactions? That is basically the same for me. Stablecoins are pegged on the US Dollar and so if I buy stuff from Steam and Amazon I could very well use my card instead.
A wise decision indeed! I guess using cards for online purchases could be more handy and comfortable than using a stablecoin! On the contrary, stablecoins specifically with Tether have known risks associated with it that is why most merchants could not be easily convinced in adopting it - one reason mainly because it has been controversial with regards to the integrity of fiat backing and until this date, the holding company of Tether hasn't provided any verifiable proof that Tether has indeed a 1:1 backing for U.S. dollar!
So in essence, if you buy goods or services using your cards, the merchants are assured it has fiat backings courtesy from the bank of financial institutions that issued it whereas if you use a stablecoin such as Tether, merchants are not really sure if it indeed has fiat backing - the reason why stablecoins would not get a wider audience and adoption in the long run. Imho.