"can’t they just make USD transfers fast and easy?"
its cause regulation???, easy and fast to moving money its good until you got hack from state sponsored actor that you can't even do anything (ehm ehm north korea ehm)
in seriousness, people can get away with billions of dollar and "vanish" is wild world if you think about it.
UK regulations forced banks to create APIs for money transfers, and UK banks now have instant free transfers: https://en.wikipedia.org/wiki/Faster_Payments
USA is just bad at governing. Tries not to tell corporations what to do, so it ends up with toothless half-assed laws that do nothing except being a tool for regulatory capture.