I'm sure that part of that is just the large difference in volume. I do agree with you, but the higher the volume of questions and answers, the thinner the 'love' has to spread. Sure, SO gets a lot more traffic so naturally it should have a lot more 'love', but a lot of people drive by just to get a bit of information when they need it. I don't know what percentage of SO's traffic by day is 'read only', but I'm positive it's got to be significant and on the rise. I'd expect the same (just to lesser degrees) for the other well established trilogy sites.
On the other hand, you have the new 2.0 sites that are started by an enthused group of people who really want the SE system to work for their topic of choice, and they have a limited amount of time to demonstrate that it can. I think it's a little early to be comparing the trilogy to the new 2.0 sites, I don't think any of them are yet sufficiently established to compare.
That being said, everyone who has not visited the site is shown a link to the FAQ, it's becoming obvious that people looking for a solution to their particular problem often just don't want to read anything that isn't a solution to their problem. Allowing for totally anonymous voting would be very problematic. I wish I could convert "me too" and "Thanks" answers into up votes for the OP cast by "Community". That alone would make a major difference. Some people do take the time to say thanks before they leave, just not in the way that we hope for.
I'm sure it's also a difference in cultures between the various communities, but I think you nailed it with drive by (read only) usage.
I think the real question then becomes "How can we better convert visitors from search engines into positive contributors?" You'd have to show them that it's worth it within a couple of page loads, and I'm not sure how to do that :)