There is a good explanation as to why the first gen leaders ROCK at governing, but the later ones just SUCK, and it has nothing to do with it being hardcoded in. Let me explain:

1) Most governing virtues actually come from BUILDING stuff, rather than sitting around existing buildings. Naturally, it will be your first gen leaders that do most of the building.
2) Even worse, sitting around fancy buildings is actually BAD, contrary to what you think. Taverns, Theatres, Arenas, City Plumbing will give you traits like drunk, corrupt, expensive tastes, girls, etc. And believe it or not, an Academy doesn't give your guy any traits at all! (If you don't believe me, do a search for "Academy" in export_descr_character_traits-triggers.txt - you won't find it there!). So your later leaders who are sitting around your capital are acutally picking up boozing and flirting tips rather than learning to govern!
3) The Corruption trigger is also activated once Treasury > 50000. It gets even worse past 100000 and maxes out once > 150000. Naturally this doesn't affect you at the start, since you are so broke.
4) Sitting around not building anything also gives vices (eg. BadBuilder, BadMiner)
5) Many bad traits are self perpetuating. Once your guy has a bad trait, it's usually downhill from there, even if you move him away from the big town.
6) Inherited traits are really quite few and far between - only some traits can be inherited, and even then the chance is quite small. If they are inherited, it will show up when your character comes of age (ie. 16years). If it's not there then, it hasn't been inherited, full stop.

I've summarised and calculated and compared all this in an Excel/PDF file. You can find it at http://www.geraldtan.com/rtw/

Cheers!

Poohbear