Makes sense to me. The plumber doesn't require 4 years of undergraduate plus graduate/dental school. I would think that the cost of that education would be significantly higher for the dentist rather than the trade/technical schools used by the plumber.

Most dentists (not specialists) usually start with a pretty steep debt to pay off first. Plus the equipment costs would be much higher.

That being said, I've taken my vow of poverty being a Catholic school teacher.