I think this is because I've read of people complaining of 3 or 4-word comments, and others stating that the only proper comment is a detailed, multi-paragraph one that shows appropriate analysis and reflection. I like those, too. I like those, best. But I frequently don't have the energy for that, particularly on a weekday night, and thus have been reading without commenting at all.
So I think I will go back to leaving brief comments if I can't summon the wherewithal to leave a detailed one. As an author, I like any acknowledgment that someone liked a story, so I can't help feeling that most other authors would prefer that to no comment at all.