There's no descriptions because Nicolas spends most of his time working on new features and adjusting old ones. Others in the community spend their time on their own personal projects, or just don't feel the need for documentation I suppose.
Either way, you can't really blame anyone in particular for AndEngine being vastly undocumented. It's an opensource project, so anyone who feels they want to contribute can do so. The funny thing is that with the number of people who've posted concern about lack of documentation, it seems that they do not feel like contributing either when they start to learn their way.
If I had enough time to setup a website purely for the purpose of AndEngine documentation/guides/etc, I would not hesitate. The website could probably generate some decent traffic and possibly some extra cash via ads.