Let's say you have just gotten a brand new computer, your very helpful, intelligent etc. and you want to join AO.
You won't get in, because nobody knows you and you won't get an invite.
You would recieve no new members who could have been very good for the community. You will not be given a chance to prove yourself.
And then there is the problem of who should be bannes and who shouldn't be banned. You can be active, follow the rules and not be a spammer but still be of absolutely no interest or importance on AO.
You could try a provisional membership to be evaluated---> again more work for the mods and if the mod in question doesn't like you...tough luck
If your type of forum is so possible and good, how come there are practically no fora like that? It would need a truckload of moderators handle the invites,the kicking and the members.