Oh yeah, before I forgot
A good reason is because of choice.
Same issue as civics and promotions.
By now, most of us know that AI understand nothing about python.
AI view python benefits as absolutely no benefit.
Thus, for things which have choices, like civics, AI will not be able to make smart decisions.
Between 2 python civics, AI will just weight them based on their XML stuff such as maintenance and choose the better one, while totally ignoring the python part.
Similarly for promotion, say one which grants the player 10,000 gold with every combat victory.
Sounds good, but when compared to Combat I, AI will just see it as 10% Combat Strength VS nothing, and obviously choose Combat I. Of course, codes can be added to affect AI decisions, so that AI will choose the new promotion under certain conditions, but what we think may be good conditions, may not always be the "good" conditions to select those promotions.
Traits on the other hand, are permanent, so AI does not have to crack their heads and think which traits to choose. Of course, since they don't understand python, they may not be able to fully maximise the benefits of the python traits by doing certain things more often.
Wonders and Projects are OK as well, since the worst part is just that you wasted hammers building something you don't really need. But I will not make something like you can either build A or B, since AI does not know that once it builds A, it can no longer build B, and end up choosing the wrong building.
Events however, fall into both categories.
There are some events where there is only one outcome for you to click and choose.
But most events do have more than one outcome, so these will be unfair to the AI.
Example:
1) Pay 100 Gold and Start a Golden Age for 10 turns.
2) Pay 200 Gold and Start a Golden Age for 30 turns.
3) Do nothing.
If the gold part is XML, and the Golden Age part is python, humans will tend to choose choice 2.
As for AI, they view it as Pay Gold and get nothing, or do nothing.
So obviously, they choose to do nothing.
Codes can be added so that if they have gold, AI will choose 2 as well, but what if AI has intended other usage of that gold, like rush a wonder?