Except the game is changing, so AI wouldn't work at all in subsequent versions if maintenance work on it ceased.
Upload saves.
Why does the AI stack up 30K troops and do nothing but siege unfortified provinces
Not meaningful and easily disproved exaggeration in the general case. For a specific case, upload the save.
while little 2K armies run around taking provinces
Has not been high up on fix list, since after all it's somewhat difficult to play against. It's not a deliberately coded behavior, however. I think the problem is that "splinters" supposed to join up with their main army stop by to opportunistically take unfortified provinces. While not bad in every case it tends to leave them vulnerable since it delays them.
It just keeps taking turns giving up and taking back the same provinces when all it has to do is wipe the tiny army.
Indecisiveness is inherent to underlying algorithm in use, changing it will take years, literally (if it would happen). One does not replace tens of thousands of lines of highly tweaked code overnight.
Or split its troops into 2 armies maybe?
Funny you mention it because it never did at all until recently to avoid attrition beyond their initial army allocation. Why hasn't it? Because as you mention, it's extremely easy to to make the AI play worse by trying to make it smarter. Splitting up your army in two means you're taking the risk of the enemy engaging one of them with his entire force, so without sophisticated analysis it's not a good decision.
Not trying to come off as hostile, your complaints are valid at heart, but in practice it's not as simple as the AI changing in a vacuum. The best you can do as a player is to upload saves where it behaves very stupidly.
