Decision fatigue may also play a role (after studying something complex the extra decision of whether to vote on it feels like work so we skip it).

This. Also after reading a more complex thing, it seems common that I'll forget to think about voting at all, since I'm distracted by thinking about the implications or who I might want to share it with or what other people have to say about it. Sometimes I remember to go back and vote, but I think most of the time I just don't, whereas with funny things the impulse to focus on the author and give them a reward in response seems to be automatic.