I think one reason is that the interview process isn't JUST designed to check if you can code. While I agree that the process is broken, there are legitimate reasons why companies put up high barriers to entry.
The problem isn't a high barrier to entry, what's tested is a separate skill set. It's like testing a carpenter's skill by looking at how good he hits with the hammer.
I would like to think we have a problem if a carpenter is unable to wield his hammer properly.
Please don't hurt the metaphors. They have feelings too.
I feel like you can tell a lot about a person by observing how they perform their craft. Swinging a hammer probably doesn't offer too much insight when done properly, when its done wrong, feels like a tip of the iceberg situation.