But, so-called “exit controls,” where documents are checked as travelers are leaving the country, were popularized last century by Nazi Germany as a great way to ensure that they could control, round up, and exterminate the Jews and other “undesirables.” It can obviously serve no purpose of keeping terrorists out, because it only affects those who are already in.
Or to put it another way, they're standard in practically every country apart from the US. Because what's the point of issuing someone with a fixed-term visa if you have no record of whether they left at the end of it?