Why do mass marketed CD's sound so crappy?

I posted awhile ago here asking opinions regarding the poor sound quality of Coldplay's "A Rush of Blood to the Head" CD. Now I want to ask the same question of U2's latest (which is great, btw). I also find Sheryl Crow's CD's to sound underwhelming and dissapointing. Besides that fact that I love her music. What gives? Are the artists clueless? Don't they hear what their releases sound like? Are the record companies deliberately turning out crappy sounding CD's to please the masses that listen primarily on Ipods and walkman's? Man, it makes it real tough to enjoy music I really love to listen to when it sounds so damn bad.

The first track on U2's newest, "Vertigo" really rocks out, but it sounds boomy and muddled. I wanted to turn this up real loud, but it just sounded awful. I'm bummed.