The problem has at least two parts:
One problem is the cost, and the other problem is that the cost no longer automatically comes with a ROI that makes sense of that cost.
Young people who are considering dentistry as a profession are looking at the neighborhood of $500,000 in education expenses. That comes along with the opportunity cost of being tied up for at least 8 years, and hence, not earning any money. In the meantime, the trend in dentistry is a reduction in earning ability, as the dental schools pump out more and more dentists, and they are, in most areas, struggling to have enough patients to pay the bills. Because they are desperate for patients, they sign agreements with insurance companies to take low fees, in the hopes of getting more patients, but since everyone is doing that, it's no guarantee they'll have enough.
Amazingly, in the face of these scary numbers, the dental schools have no shortage of applicants for their incoming classes. It's scary.