For the posterity of this thread American banks are no longer required to hold any reserve requirements at all[1].
Though I guess that is a "different" issue depending on who you ask here.
Ask yourself this - can everyone in the country take their money out at the same time
For these shitty stablecoins: nearly everyone
For the actual money you use everyday: Maybe 2-3% of people can cash out of the system properly.
A system built on trust works until it doesn't.
[1] https://www.federalreserve.gov/monetarypolicy/reservereq.htm