Why don't airliners carry radar jamming technology or missile counter measures?
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.