In addition to factors already mentioned by other users, I believe that there are also social/cultural reasons for that lack of engagement.
Commenting in Reddit is like stepping on a mine field - no matter how innocuous your comments are, you're bound to have users there assuming words into your mouth to screech at you. Plus all the "ackshyually", one-upping, "wah TL;DR!" (i.e. "I'm entitled to an abridged version of what you said, even if you likely spent far more time writing your comment than I would reading it").
Eventually you say "why bother commenting? Just to get a headache?" and stop commenting altogether.