Hard to judge... Depends on alot of factors, including difficulty settings, defensive pacts, and other empires guaranteeing your independence...

If you have defensive pacts with, or other strong empires guaranteeing your independence, the AI might consider it too risky to attack you, even with no fleet of your own, as it would mean war with all the other empires too... Combine this with lower difficulty settings, where the AI is less aggressive anyway, and it's entirely possible that this is working as intended...

Grand Admiral, broke all my defensive packs no guaranty. Multiple games, borg, federation, klingon. AI never wars

Ive noticed the same thing. In order to goto war, I have to start it.

Even with canon wars enabled. Nothing, only way war starts is if I start it

I can't really comment, since I rarely have an inferior fleet to my neighbours, so I've always just assumed that it was due to my fleet strength, etc...