Someone just asked why...
1 - AIBU is fighty, if someone is upset or says "please be gentle with me" they will get a much nicer response if they are posting in a more appropriate topic.
2 - We have a lot of sections that were once requested but now are very quiet. This is a vicious circle: the more people post things in AIBU that should be in another topic, the quieter the other topics will get, and the more people will post in AIBU because the other topics get no traffic, so there is nothing being posted in those topics, so no-one posts there, etc etc.
3 - Some people seem genuinely unaware that there is, for example, a Movie Reviews or Adult Fiction topic and when they find out they are quite pleased to have a place to browse movie threads or book threads