Second, the IRS processed 247.8M total returns[1] and 3.59B supporting documents[2] in 2017. While I think the IRS could do a better job (like, well, the entire rest of the US federal government), what other tax system processes anything near that volume?
Also, I'm not buying "just about every other country". Not even close. For starters, I find it hard to believe that more than a handful of African countries manage to do it.
[1] https://www.irs.gov/pub/irs-pdf/p6292.pdf, page (3)
[2] https://www.irs.gov/pub/irs-pdf/p6961.pdf, page (4)
The volume doesn't really matter, because it scales linearly with population - a larger country just needs more bureaucrats, but it can afford them because it also collects more taxes from more people. There's nothing unique in that regard in US.
But if you want something in the same ballpark, we can look at, say, Russia (population of 150 million), or even at Nigeria (200 million).
I don't know why so many people in this site seem to think this is a trivial thing for the IRS. 100% review after the fact isn't even feasible right now, let alone having the information in January.