I've had this problem for years, since before Rebellion came out. I can't remember the last single player game were the CPU players actually put up a fight. I get attacked occasionally for the first 15-30 minutes, and then all the CPU players stop doing anything. They stop attacking me, each other, and usually stop building fleets entirely.
I play free-for-all, either 4 player or 8 players. Problem happens on all maps and difficulties.