I think some posters forget that they are not just chatting to the posters in that thread who are also oversharing, its not a closes board. A thread with 5 posts could be read by 100 people! On open forums there are many many more lurkering members than posters, and many non members also browse.
I also think people forget how easily jigsaw pieces can be put together. If you put a little bit of info on one thread, and a little bit of info on another.... it can add up to quite a big picture!
Some posters share A LOT! where they live, what companies they use, when their homes will be vacant.
Post on one thread your county
Post on another thread that you need specialised home insurance e.g. for a thatch.
Post on another thread that your village pharmacy just closed.
And that can be enough for someone to recognise your exact house!
Then you go and post elsewhere personal info about your DH, friends and colleagues too!