For one, this is a society that loves certificates, degrees, and any type of accreditation. When comparing dentists with doctors, because the degree is perceived to be not as prestigious, dentists are not afforded the same esteem that doctors might be.
Typically, doctors spend a lot of time in school and then spend a lot time in their residency before going to practice medicine. A dentist, while the training might be just as rigorous, does not spend as much time in school, depending on the program.
Then, another reason that dentists might not seem hold the same respect is that they typically do not earn as much as doctors. Salary is always a factor in a society where people are often judged by the positions they hold and the salaries they bring home, and this is also the case. The interesting thing is that there are all types of dentistry, and the field can encompass a number of areas and pay scales.