I think those who have completed their families also like to partake in the excitement of other people's linespot threads, and also giving advice because they have obviously been there already and perhaps have a lot of poas experience. I don't think they are all stalking poas threads because they still yearn to do the same, although there could be some who do but it doesn't matter why anyone is on a particular thread as long as they are trying to be helpful which everyone seems to be.
Also, some who have first thought they completed their family (like I did) may change their mind later down the line as well, things aren't always set in stone.
If I knew for sure we had completed our family I would likely still be looking at the pregnancy test section trying to be helpful because I like seeing others get their positives and getting involved in their happiness because it's nice, and also it's a general interest to me anyway (human body, pregnancy, birth, all of it)