most are pretty obviously from the same digitally-limited/brickwalled master
Actually it makes MORE sense to use dynamic range compression for an analog medium due to its smaller dynamic range compared to digital.
My understanding is that heavily, overly compressed rock music is a byproduct of how FM radio advertising money is distributed
When people put their iTunes libraries on shuffle, or make mix CDs, tracks from different releases get played side-by-side. Unless you have put in effort to ReplayGain everything (which most people aren't going to do), a dynamic track is going to sound too quiet.
I think it's got very little to do with the radio.