you'd collect interest on money you loaned out. they'd have to be loaning out some amount of money, and even if it's just 'backed up' by all this 'inflight' money, that's not terribly safe. I think the majority of the slowness is just down to old/legacy systems, and the sheer volume of stuff that has to be touched to make upgrades. I'm not saying upgrades/changes
never happen, but it's a big effort. Why do we still have relatively poor security around a lot of financial stuff? Because the cost of just writing off fraud is still usually much cheaper than the long-term effort of systemic upgrades.
Other countries didn't have as much infrastructure to upgrade, and have been able to leap frog the US in many ways.