Probably a part of it is that player characters tend to skew old and high-stat and warlike.
- If you're old or have a disease, any blood duel is dishonourable, so probably the AI is blocked from these challenges (except in exceptional/event-driven circumstances).
- If you have PCS>50 (which is not hard: I routinely manage 100-150), that's better than almost everyone else in the world, so a normally-cautious AI character probably wouldn't challenge. (Brave/ambitious might override this.)
- Players are probably more warlike than AI characters (I often dip out of all wars so I can duel/etc, then declare war again a few days later). Most types of duel require that both parties are at peace, ie. being at war blocks AI from duelling you (except for specific event chains etc).
NB: This is all speculation on my part - I believe the details of who the AI chooses to duel are fairly hard-coded, so we probably will never know for sure.
Also, I have definitely been honour-duelled in warrior lodges. And also insult-duelled after winning a warrior lodge event-chain contest (and insulting the loser). I haven't ever been blood-duelled by another warrior lodge, but that's probably due to either being at war or having such high PCS that I'm not eligible to be picked.