There are a few reasons:
* Lost of listening is still done on the radio and that means it can happen in noisy places. Dynamic compression is a huge help in making the sounds of the music audible in noisy environments.
* Some producers are still fighting the loudness wars.
* At concerts an other live-sound events, compression is an essential tool. Screeching vocals and guitar riffs would be super unpleasant. Dave Rat (sound guy for the Red Hot Chili Peppers) uses a *ton* of compression at their live shows. Check out his YouTube Videos, they are fascinating.
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