I don't think I've ever seen the AI do this - the economy and empire is always developed.
Now defences on the other hand, those are indeed a problem. Most planets have few to no defences, and even when playing TEC Enclave the AI doesn't garrison its worlds. An extra free 500 supply + 500 supply from all neighbouring garrisoned planets, would really change invasion dynamics!!!
I don't think I've ever seen double starbases either. Nor starbases outside the AI's own worlds. When playing with friends we sometimes stack starbases at critical points e.g. one star with something like 12 starbases deployed to it.
The AI also seems to fail to use the Advent's Unity powers, or is it they don't spend the points?
I think the bigger issue, especially on higher AI settings e.g. Impossible, is that the AI is locked into a Zerg rush strategy and never really builds defences meaning you need to be able to either outproduce the 'cheating' AI, else tech up and lockdown invasion routes, but building up for a 3,000+ supply invasion isn't easy!