Most apparently don't believe this is possible or worth their time.
Or perhaps they would rather their patients come back with cavities so he/she can charge more to fix them? Here's an experience of mine that's relevant: I once had a dentist who told me I had a cavity and showed me a little black spot on one of my teeth; for whatever reason I declined that day and said I'd come sometime in the future when it was convenient, to fix it. In the meantime I was introduced by a friend to another dentist who was recommended because he was much cheaper, and so I went to him to fix the cavity - to which his response was, "that ain't no cavity", and promptly scraped the spot away with a pick. He also did an x-ray and showed my teeth were in perfect condition with no signs of any decay. This was a few years ago and I've not had any problems with my teeth since.
Or perhaps they would rather their patients come back with cavities so he/she can charge more to fix them? Here's an experience of mine that's relevant: I once had a dentist who told me I had a cavity and showed me a little black spot on one of my teeth; for whatever reason I declined that day and said I'd come sometime in the future when it was convenient, to fix it. In the meantime I was introduced by a friend to another dentist who was recommended because he was much cheaper, and so I went to him to fix the cavity - to which his response was, "that ain't no cavity", and promptly scraped the spot away with a pick. He also did an x-ray and showed my teeth were in perfect condition with no signs of any decay. This was a few years ago and I've not had any problems with my teeth since.