The AI can't handle a war with a civ on another continent very well. Many fake wars have to do with being on different contintents. On a pangea map the AI is much more capable.
The AI declaring war to someone is influenced by many factors: personality of the leader, diplomatic modifiers, room to expand,...
So in certain games you will see the AI's declaring war on you and eachother very much, while other games can be completly peaceful...
In my opinion, there is nothing faulty with the AI's tendency to declare war or not. Every game is different...