One explanation I've heard (don't remember which news source) is that dentists just don't have as much demand as in previous decades.

Due to improvements in hygiene, and public health measures such as water fluoridation, children get fewer cavities and adults lose fewer teeth ("having an insufficient number of teeth" was once a major cause of men being rejected from the military draft).

So dentists have had to pivot from providing necessary care, such as fillings and extractions, to more discretionary services such as whitening and other cosmetic procedures (oh how my last dentist used to try to push Invisalign® on me at every visit!)