Indeed I have wondered the same thought. Given the experience required to be hired in North America by an airline who operates the A-320 or B 737, the new hire would have flown with at least one or more previous airlines or commercial operations. During that earlier period any dangerous tendencies would have been noticed.
Apparently not, remember the Fedex screwup and the Jetblue nutcase.
Anyway, the industry is different in europe to north america. In europe there have always been cadets entering directly into airliner flight decks. And airlines have run their own schools since WWII, therefore there is a lot of experience with training from the ground up.
Interestingly enough, every winter those cadets do fly in north america as well, in 737s too, not to mention those lucky cadets that make it into long haul early on and fly over there with their widebodies.