Are you sure they didn't attack you because you had a place they needed, etc? Or had a lot of trade spots? Or were generally too powerful?
I can give you a definite "no" to your first two questions, and a qualified "no" to the third: In most campaigns, I hadn't yet gotten anywhere near the point of being big/powerful enough where all the other factions would gang up on me. And even in the 2-3 campaigns where I *did* get that big, the problem was that by the time the remaining factions dogpiled me, I was literally already too powerful to be stopped.
In fairness, however, this isn't a problem unique to Empire: Going all the way back to MTW, the AI-controlled factions often wait too long before they band together to take on the "Big Guy" -- regardless of whether the Big Guy is me or another AI-controlled faction. If the BG is an AI faction, it often falls to me to fire the first shot in pulling them down (in which case the rest of the factions usually follow suit). If *I'm* the BG, though, the other factions almost never get their act together in time; by the time they do, it's already too late.
But I digress. Another game that I thought where the AI is actually pretty decent is Spaceward Ho! IV (for Windows). Of course, it's a very simplistic, "beer-and-pretzels" space strategy game, which certainly doesn't hurt (the simpler the game, the better the AI often is). Regardless, the AI has always been fairly good at scouting out enemy planets, assembling decent fleets, and exploiting any weaknesses on the part of their enemies -- I can't tell you the number of times I've lost under-defended colonies to enemy forces that I didn't even suspect were nearby!
