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.
You have to remember this doesn't sit within the realms of healthcare nor NHS dentistry but rather cosmetic treatment and if those vain enough to have these unnecessary treatments are turned down by one (ethical?) practitioner, they'll just find another willing to cave in. Pretty sure Rylan went in to his screeching 'as outrageous as possible please'.