One other factor...it's expensive to send out pictures and letters. I am sure he gets thousands of requests, and as wonderful as it would be to THINK he has a moment of time to read the letters, well lets be real. I think the best thing you can do as a fan is respect his privacy and appreciate his work
Nic does appreceate his fans but as fans we too have to appreceate the fact that not only is he very busy with his working schedule but he also has a personal life too.
Also, Nic's work takes him all over the world which makes it very difficult for him to recieve, let alone read, fan mail.
I sent a nice gift to Nicolas Cage via fan mail several months ago and there was never any response back or anything. Not even an autographed picture. I was very disappointed as I thought he cared about his fans.