Stuart,
I've poised this question in the past, but never got any response.
Here is my take on your question:
It's not compulsory to post sightings.
Could be some of the members are disabled in some way and cannot post.
Some people prefer not to get involved but prefer to read postings of others.
Some members have in the past posted sightings and their word has been questioned (fact), thereby making them think is it all worth it.
Some members may not have a computer.
Where some members live it is maybe well reported so why should they bother.
Some members may think the forum is for twitchers only. ( IT ISN'T )
Some members may think that reporting common birds is frowned upon so don't really know what to do for the best. ( If it's any help I report ALL sightings).
And lastly until I think of some more reasons, when I first joined one well known member told me "we don't want sightings of Starlings unless it is a massive amount, or a Robin etc" , I took notice for a few years and then decided to ignore this stupid piece of advice and since then report all birds! Both this forum and the BTO want all sightings of all birds no matter how common.
Hope Stuart's thread gets those members who never post sightings to reply or at least the odd one or two.
If those who Stuart is aiming this at prefer then send him an email or PM, that way he will get answer/s in confidence and put him out of his misery
Kind regards
John aka Max