I've seen the posts from a few people complaining about this issue. This is what I found:
Gigantic/ loose clusters/ challenging/ 9 races/ terran w/ 30% social production. After about 70 turns I noticed that I had 12 colonies and everyon else had on 1 or 2. Hmmm. This is less action than I've seen on "beginner".
So what do I do? Instead of throwing my hands up and writing an angry post I actually restarted the game with the same settings...not ctrl-L...I restarted the game entirely.
This time I got much better behaving AI. As usual, the Torians and Arceans are expanding quickly. Their research seems to be up to "challenging", and I'm meeting them by turn 40 - 50. (met the Torians at # 25, i think)
I did this a few times and I've come to the conclusion that SOMETIMES the AI either doesn't initialize properly, or it just gets "hung up" and really expands s-l-o-w-l-y.
This wouldn't be a problem except that it can take you 50 - 80 turns to figure out if you "got a good one", if you catch my drift.
It's not that the AI is dead, it's that is dead SOMETIMES. I think a lot of the "AI is dead" complaints are only partially legitimate.
Maybe too many CTRL-L restarts are the culprit

?