Because of several reasons.
1: This one's a little personal. I gave up on AI quite a few years ago when the gal I voted for gleefully didn't win. And this was the final vote people. The final vote. Pfft. I felt like AI had done this to me:
2: We're running out of talent. And when I say "talent" I mean pure, internal, uncultivated talent. Now, notice that uncultivated has been underlined. Twice. I'm not an idiot. Talent needs teaching, needs to be drawn out. Especially if you're going to be on national (and international?) television. And Youtube. Definitely then. I'm no musical/vocal expert, and I admit that willingly, but I've been around longer than AI. And I've heard real talent, both there and in other places. And I've seen Phantom of the Opera. Everybody needs an Angel of Music. Except me. Because 1: I know I can't sing, and 2: I just want an Erik.
3: This sort of goes along with the whole talent thing. Some people need to admit -- or at least acknowledge -- what I had no problem doing: they CANNOT SING. I'm sorry people. But national TV is not where you want to learn that lesson. Especially where Simon Cowell is judge.