[bswen]

Because there aren't a million online tutorials for learning dentistry and you need more than a computer and an internet connection to start learning.

[crpatino]

Basically, the problem is that no sane person will let you mess with their teeth unless you lack the proper credentials, and experimenting on oneself can be painful enough that almost everyone will quit after their first equivalent of an off-by-one-array-access segmentation fault.