I had wondered myself; now I wonder no more.
Q: Dr. Bernstein, why have you not had a hair Transplant? I’m sure this question gets asked a lot. Thanks.
A: Yes, all the time. My donor area is very thin (so I am not a good candidate) and I have gotten used to being bald. It has been years.

I’ve added a new link to the sidebar: The Bernstein Medical Hair Transplant Blog. The blog is run by Dr. Robert Bernstein, Associate Clinical Professor of Dermatology at Columbia University in New York. Bernstein posts in an easy to understand Q & A style covering hair transplants from A to Z.
Here’s the best recommendation I can make: I have no interest whatsoever in a hair transplant… and I’m a subscriber to his blog.