True, but sometimes people need a poke - people sometimes have things that are stressing them out. I have an entire roleplaying game to babysit, and I know that might not be hard for some people? But it's a lot for me. Also, that's not just two threads for everyone - it can be as many as six for some. And often people do threads to convenience other players and give them a chance to do something.
No, it's not your responsibility to remind anyone, but if you've been waiting and your anxious, don't be passive about it. Give someone a poke. It makes everything better in the long run because they got a reminder and now they can remember to do it sooner rather than later. It really is better to just poke someone if necessary.