I do hate this 'why do people get over-invested' bullshit. That's the point of a message board/support forum to talk and communicate, and support each other, and some posters DO like to stay with the thread and make sure the OP is OK. Coz shocker some people are actually quite caring and nice.
That said, I am sick to death of threads with FAKE content, where the OP reels posters in, gets them to spend time and effort and energy helping and supporting them, only to see the whole thread deleted, and all their helpful and supportive posts deleted too.
I have spent quite a bit of time composing a post which was helpful and supportive and took me 20-30 minutes to write, and then the thread vanishes before I click post, with 'TROLL' or 'PBP alert' put on from mumsnet. (OR it goes within 5 or 10 minutes of me submitting my post.).
Or sometimes a thread is deleted, and you get the infuriating message that the OP decided the thread was too 'outing' or identifying, and requested for it to be removed. (I suspect they didn't like the replies they got!)
I sometimes think, why did I even bother to reply? 
So I am tending to submit supportive posts now, but not spend more than 5 minutes on any one particular post (on these types of threads, where the OP has a 'dilemma.') Because of the risk of the thread vanishing into the ether!