My experience is different. Here are examples.
Here is situation. It is now 2-nd turn after Parthia and my empire have jointly conquered 3-rd Empire (it's name is not even worth to be mentioned anymore). Parthia is surrounded by my Empire on all sides, except for Parthava, which is a single region satrapy of Seleukidai Empire. Parthia is afraid to attack Seleukidai. So I am. Even considering that Seleukidai is currently engaged in several wars, two of them are with powerful empires.
Parthia clearly understands that my empire is natural and the only next target.
I clearly understand that Parthia is my next natural target, because I seek to secure my eastern border. My northern border is secure already.
Parthia understands that I will attack it eventually.
So I expect imminent attack from Parthia every turn now.
And please notice how Parthia is preparing to pounce.
I estimate Parthia has another army near my eastern-southern border. If Parthia attacks within next 5 turns, I will struggle to keep several regions in my North.
This situation is not single example of AI concentrating their armies.
I've seen several concentrated armies of Seleukidai, when we were fighting common enemy. When I was fighting Arche Bosphorus, it brought 1 army to threaten two of my regions (capital and recently conquered). Then it brought 2 more armies to conquer my recently conquered region.
So in my experience AI is performing very good strategically.
The topic starter, in my opinion, is missing some important facts. Like maybe his target is fighting other wars now? And therefore his armies are on other fronts.