At least in the US its because dental insurance isn't part of normal health insurance. Dental insurance mostly just covers the bare bones (teeth?) treatments like regular cleanings and an xray every couple of years. SO in order for the Dentist to make money they ahve to upsell every other treatment not covered by insurance. With regulars health insurance the doc is payed for pretty much any specific treatment they deem necessary so they know they will be payed.

I think if we had the 'pay out of pocket' kind of thing for other health care we would see the same thing.