Yep. I ride mostly in the woods, but I've witnessed the same thing.
I always try to make eye contact and say "hi" and "thanks" when they yield trail. I kind of assume it's part of my responsibility as a cyclist to maintain good PR. I've formulated a few theories as to why good vibes are not returned, to wit:

-They're scared of or intimidated by me (people are paranoid)
-They feel like the outdoor experience belongs to them, and I'm infringing on their enjoyment (people are selfish)
-They're like the roadie in my neighborhood who waves when I'm on my road bike but not when I'm on my mountain bike (people are a$$holes).

I will never stop trying to be a nice guy, but I don't get disappointed any more if people don't return the gesture. And there are lots of nice folks out there who *do* smile and greet, which makes it worthwhile.