Here's the ugly secret. The Alaska Airlines MVP line, reserved for their frequent fliers, does offer call backs. I just got off the phone with them. The wait time was about six hours, but I was able to do something else in the interim and I was not driven insane by their corporate theme music.
So there's no technological reason why Alaska Airlines doesn't offer callbacks on their standard customer service line. It has to be a conscious choice. Why would firm chose to make life so uncomfortable for their customers? While I can't prove this, I strongly suspect that the system is deliberately designed to be inefficient for the tier of customers who call the standard service line.