When reasoning about risk mitigation, it's important to keep in mind a few numbers:
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I would like to see a comparison of the number of commercial passengers killed, over the past 10 years, say, by suicidal/homicidal pilots versus the number killed over the same time period by, say, falling down the stairs and breaking their necks while boarding at non-jetway airports, or by being struck by vehicles in the passenger pick-up / drop-off area, or from allergic reactions to some food consumed on board aircraft.
it's entirely possible that, compared to other risks facing the travelling public, deadly action by insane cockpit crew is not actually very high on the list of things to worry about, and may not warrant complex or costly solutions that in turn introduce other risks.
You should also throw into this calculation the most dangerous part of any air journey: the drive to the airport.