It has been a perplexing phenomena in my life as well, although, besides my wife, I have not asked for proofing or giving feedback on a book's draft. It is a lot to ask and I have my own deadline.

Besides, a friend asked me to do it once and I gave honest and, I truly believe gentle feedback, as per requested. She never talked to me again.

What does get me is that few friends and family have bought any of my books. And when they do, it is hard to get them to write a review. I make it a point to do so with my author friends. I think it is like you said, they do not understand the importance, and how much it means to us a writers.

Sure my books of poetry and spiritual writings are not for everybody. Still...

Anyway, good luck and happy writing with your book. Cheers.