Anti-hijack measures

First, my very deepest condolences to all Americans for what has happened.

My question is a pretty simple one. Is there any reason why commercial airplane cockpits can’t be automatically sealed off while planes are in the air?

I’ll close this thread since this subject has been discussed very recently. I refer any remaining unanswered questions to the earlier thread, which started in GQ but was moved to GD: Can we seal aircraft cockpits?