Unfortunately it's not just celebs that do this to themselves. I know someone who has an American friend who looks ridiculous. She has a face full of botox and teeth so big and white Jaws would be scared of her. That's what vanity does to you sadly.
Vanity and also greedy dentists. Some dentists will often agree to a veneer procedure when in a lot of cases a basic bleach whitening of the teeth is all that's needed.
Always get a second opinion when doing anything cosmetic with your teeth.