Which types of aircraft are required to carry ELTs?
Everything you need to know (at least in the USA) is here.

Regarding airliners, it appears that they are carved out of the general requirement to have an ELT in 91.207.f.2.   I guess that makes some sense.  If you're flying a scheduled flight, presumably we should be able to find the plane, since we know the takeoff time and approximate airspeed, and we have radar tracking by ATC/ARTCC.  Unscheduled (especially VFR) flights are where things get dicey.