Because you will keep the perfect mp3/flac rip you downloaded and not pay for it. Except you are going to say you are the person who always pays for everything he keeps...everyone always seems to somehow become altruistic when it comes to this point of discussion on chat boards.
Read the post you replied to, where I said I delete stuff I downloaded that I dislike.
I pay for the things I keep. Why would I keep things I don't like? That's just wasted disc space. Why assume I don't pay for things I download? You say they're perfect flacs or mp3s but ripped content is often far from perfect, so buying has the advantage of keeping me legal, giving money to people I like and getting me a better copy of the file.
I don't see any difference between downloading then deleting music and asking to listen to something in a record shop.