Aside from the weight and expense issues, consider that an ejection is a very dangerous thing to do... for example, the ACES 2 seat used in most US military aircraft has around an 85% success rate. The definition of success is, the person survived, quite possibly with injuries.
Current ejection technology requires the person to be tightly strapped into the seat with a full six point harness.
An ejection is a very violent event. You get slammed with 20G's when the rocket goes off. If you're not in top physical condition, as military pilots are, your spine will be damaged. If your head isn't held perfectly inline with your spine, your neck will be fractured. If your arms or legs strike any part of the aircraft on the way out, they will be ripped off. And you still have to make a parachute landing, where you hit the ground at about the same speed as jumping off of a fifteen foot ladder. Many military pilots report a loss of one half to one inch of height after an ejection, due to compression of the spine.
And freaky things can still happen. When Chuck Yeager ejected from an out of control NF104, he separated from the seat, and then slammed into the bottom of the seat, where the burning residue from the rocket motor set his flight helmet on fire. When an SR71 was lost while trying to launch a D21 drone, the RSO (back seater) died when he landed in the ocean and drowned.
So, imagine the average civilian, blasted out of an airliner. Their neck is fractured, they probably lost an arm or a leg on the way out, they slam into the ground and fracture their pelvis, and the parachute catches the wind and drags them across the ground for some distance... if they survive all of that, they'll die from their injuries before rescue teams can arrive.
Ejection is a measure of last resort in a military aircraft that can be expected to suffer battle damage, for a military pilot in top physical condition, tightly strapped in, and trained on what to do, when it's either maybe get killed or definitely get killed if you don't eject.
For the average civilian, in the safest form of mass transit, it would just be a different way to die.