I agree in a perfect world it would work that way, but the way I look at it you still more or less get that even with "cheating" buffs. Say an AI has 3 planets perfectly developed and as a result have equivalent fleet to you. Now take that AI that has 3 planets but because of shortcomings they are poorly developed. However, because of cheating buffs to the resource production of each planet they are still able to produce an equivalent fleet to you. When you go to war in either scenario taking one of those planets is still denying them the same chunk of resources. If they lose a planet they also lose the ability to have the resources they produce on that planet receive some sort of pity bonus. So the strategy is the same, take the same worlds to reduce their economy and win the war.
It's not ideal, but for me it works better than tearing your hair out pining for a great AI that has never existed in Stellaris and most likely never will.
Well think of it like chess against an AI. Virtually any chess AI these days will win every game against almost any opponent, except the very best. To counter it we reduce it's ability to predict and maybe introduce a little variability and randomness to it's actions. It's still playing the game though, so as a human I can observe all the moves it makes, understand how and perhaps why it makes them, and maneuver my assets strategically. What we don't do is turn down it's difficulty and then have new pieces spawn for the AI to compensate. That would take all the strategy out of my game, because I wouldn't be able to put together and execute a strategy based on the board state. Instead, my strategy would change to respond to the spawning mechanism that actually poses a threat, turning the complex game of chess into a one dimensional game of checkers. Yes, it's still a game, but it's not the game I want to be playing.