I think it mostly has to do with the people who create the audio for the commercials themselves; how it was recorded.
I remember, quite a while back, there was a poster here at AH who said that was his job. He said he tried to tame down the compression some, and that he nearly lost his job for even attempting to do so.
The companies that hire the ad agencies want their commercials at maximum compression.
If it bothers you, use a DVR and FF, or just hit the mute button. You can also set DRC to maximum wherever it pleases you, in the cable box, receiver, etc.