>Our neighbors drive from California to Mexico for dental work. Often. They have Medicare, additional coverage from a union, and Kaiser.

As mentioned, dental coverage in the US is typically decoupled from medical coverage. (This is the same in Canada.) Your neighbors go to Mexico purely because most things, not just dental work, is cheaper there, as opposed to said dental work not being available immediately. Very different from why Canadians typically travel to the US for medical care, which is either a) to avoid long lines (and legislative prohibition of private clinics to pay more for quicker access), or b) expertise that isn't available in Canada.