Why is the AI such an idiot?

[Dovahkiing]

Every time I go on crusade, the AI shows how it's too dumb to live.
When my army arrives at a province in order to help the papal army besiege the infidels, my fellow crusaders promptly move out, leaving me alone in the siege.

WTF?

 

[nyah]

Does your army cause attrition?

 

[Lord Finnish]

I never understood why the AI raises only a tiny fraction of its levies to war. When you beat their main army and start sieging their cities, you will find most of their holdings will 100% levies which now only to serve to delay your sieges rather than stop them.

The AI is much better than before, though.