That's a good question and the answer is that the music industry sells overpriced albums filled mostly with filler while the file sharers "steal" (your characterization) "A" sides of (nonexistent) singles. "85% of everything is crap", once said Theodore Sturgeon.
Feel free to substitute whichever term you want for the word "steal". But I've always considered it to be an accurate description of what occurs when somebody acquires a product or service for their own use without paying an agreed-upon price to the producer and/or rights-holder for it.