There's a high degree of variability in dentists in large part because it's a rare job that requires knowledge and experience as well as dexterity.

I had a conversation with a guy that is a leading endodontist that trains many dentists in the area and he said a lot of the young guys are just not that good. And that the good ones really stand out. It's just difficult to find people that have the smarts, intuition, and physical talent to do the job well. Experience really helps in this field. And yes, there are dishonest dentists that do unneccesasry work so best off getting a second opinion for anything non-trivial if you don't trust your dentist.