Thereare SAMs and there are SAMs. The SA-2 and SA-3 are the real deal and sure could hit an airliner but they are a also fairly massive affairs requiring lots of maintenance and skill to use, as well as spare parts and specialist technical knowledge and a target acquisition radar. A scarebus may be big but it is small at 33,000 ft.compared to the whole sky.
The shoulder fired stuff is great for scaring folks and can hit airliners in takeoff or approach regimes but can't reach them at cruise, obviously. I don't know the size of the warhead of the Stinger or Redeye but do know that the Russian copy of the Sidewinder, in whatever configuration it was in then blew a lot of tailpipes off of Israeli A-4 Skyhawks without actually shooting the aircraft down during the 6 day war.