Because Paris Hilton is very rich. And Paris wanted to sing, so, Paris' chequebook let her. I'm not saying I agree with her "singing" (loose term for it, really), but, I also seem to remember her album failing
That's just the other end of the spectrum though. Unfortunately, our society idolises you if you're A. Pretty, but not very good at what you do (other examples include Leonardo DiCaprio) B. Sort-of good at what you do, but "dis-advantaged" in some way (race, sexuality or in this case, appearance) C. ACTUALLY decent at what you do.
A and B are sad and I disagree with them. Had the topic been about people that fall into the A category, like you're bringing up Dave, I'd express my disagreement to them as well. However, it wasn't.
I dunno, I just think that if she had been on the radio, I wouldn't have gone "that's a good voice" I'd have instead gone "who is this... she's a little ****". I dunno. I just overly disagree with the superficial element being bought into play, and the way society reacts to seeing someone like her.