why doesn't America have exit control for passports?

I've never understood this. Passport control on entry is the longest of anywhere I've ever been, and security when entering the ticketed zone is some of the worst/longest I've experienced, but passports are never checked when exiting the country. Why doesn't the US consider this important? It's the only place I've been (including countries in Europe, Asia, and South America) that has no passport check/stamp on exit.