Weak AI.

You know, there's a movie called "When Trees Were Big." The title means that in childhood, when we were little, trees seemed very big to us. But when we grew up, the trees no longer seem as big, even though they have grown too. The same goes for the game. I suspect that there is probably no secret, and simply your skill level has increased significantly since then, while the AI's skill level has remained the same.
If you are not satisfied with this explanation, let's look at a specific map and a specific save file, and we can analyze whether the AI is playing normally or not.
By the way, when I started playing (and, it seems, other beginners too), I always played on "normal" difficulty, or rather, I just didn't change the difficulty setting that the game provided. Later, I started choosing the difficulty myself, according to the difficulty set by the author of a specific map. You know that difficulty not only determines the amount of resources, but also the AI's skill level, right? If you try to play a difficult map on "easy," you will definitely see this in the battles.
In conclusion, here are two suggestions for you:
1) try playing not on standard maps, but on custom maps with really high difficulty;
2) if single-player maps still seem too easy, play against human opponents.