>I suspect a lot of people who fail these interviews would also make pretty good employees, but I'll never know.
Why don't you take a leap of faith once in a while on someone who hasn't done well? Especially if you ever interview interns and have a bunch on that team, that's a near zero-cost gamble for a large corp.
Well, the answer to "why" is "they don't put me in charge of the hiring process."
Related, interns are a fantastic way to hire because you get WAY better information about them. Instead of an hour or two of riddles, you've got a three month work history which a couple of trusted employees have witnessed. That's WAY better signal than any whiteboard interview problem could possibly get you.
Why don't you take a leap of faith once in a while on someone who hasn't done well? Especially if you ever interview interns and have a bunch on that team, that's a near zero-cost gamble for a large corp.