In the U.S. healthcare plans typically do not cover dental. In particular, neither Medicare nor Kaiser provide dental coverage AFAIK. Dentists are quite willing for patients to pay for services directly. They'll typically charge less than what even insurers reimburse as insurance billing is so time consuming and therefore costly. There's not much negotiating as when you pay directly you're pretty much paying market rate, unless you live in a rural area where there's little competition--i.e. no market.

Market rate in Mexico is undoubtedly lower than in the U.S., though. Speaking of Mexico, they seem to have a pretty solid public healthcare program. Maybe expectations are lower, maybe people are sufficiently contented by access to relatively unregulated pharmaceuticals and medical practices regardless of poorer outcomes, or maybe griping doesn't make it across the language barrier to American news, but it definitely seems like there's less anxiety about healthcare in Mexico.

>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.