Why don't airliners carry radar jamming technology or missile counter measures? Why is it that commercial passenger aircraft that fly through dangerous areas don't carry some sort of electronic counter measure to defend themselves from missiles?  Between terrorism and the various war zones dotting the globe, it seems like it would be prudent for airliners to have them.  At least for aircraft that are flying in places that are possibly dangerous.

I've been thinking that it's probably a lot of expense and weight for something that isn't terribly likely to happen, but I wondered if there might be more to the answer or if, at very least, someone could add some details to my assumption.  A good cost/benefit analysis would be appreciated if anyone is familiar with the cost of operating such a system.
Surface-to-air missile attacks on airliners at cruising altitude are vanishingly rare: there have only been three known attacks in the past half-century (MH-17, IR-655, and probably S7-1812).  Any protection system needs to take that into account -- any safety system needs to be justified in terms of cost, weight, and safety -- it needs to save more lives than it costs.


Direct jamming.  Suppose you stick an AN/ALQ-99 jamming pod on the airplane.  That's 6000 extra pounds, a good deal of drag, and a bunch of parts that can break off, catch fire, or otherwise be a hazard.
Active countermeasures.  Stick an AGM-88 HARM missile on the plane.  Now, your anti-radar missile isn't much faster than the radar-guided missile trying to shoot you down, so for reasonable safety, you need to launch as soon as you get painted by the tracking radar.  What do you suppose the odds are that a twitchy co-pilot acting as ECM/weapons officer is going to launch against an innocent control tower?  How many weather radars are you willing to destroy to save that one airplane every 20 years?  Are you sure that missile will never malfunction on the launching rack and destroy the airplane?
Stealth.  Hiding from radar-guided missiles means you're also hiding from traffic-control radar.  How often are crashes prevented by that radar?  I bet it's more than one every twenty years.


The benefit is roughly one less airplane crash every 20 years.  That means the cost, both in terms of money and in terms of risk, needs to be very, very low.