The main reason is that the captcha is broken and Clem feels that phpBB has so many problems in itself that he does not want to add some mods - there are some nice mods if you dare install them.
We use flood control but I don't like the rest of your suggestions (like Limit the number of posts per day - on a busy day I can post up to 80 posts, so I'd be spam trapped
We are of the opinion that once logged in you are logged and if the spammer bypassed the captcha to get registered they can surely bypass another captcha
Akismet would be an idea but honestly I don't know how to implement it
We don't get spam now (the odd post perhaps) and partly this is because I go through all new inactive users and delete those that are clearly spambots - anything with p orn or v iagra or buy
They seem to register and wait a while before posting
If we get lots of this crap again we have to seriously reconsider but for now no guest posting has done the trick for us