Well, it isn't so much that computers are bad as computer AIs are bad at games. I have been mixing it up lately, playing some Starcraft 2 against humans and some against computers and I still find it shocking just how terrible the computers can be despite their superhuman ability to control everything at once. As an example, there is an achievement to fight 4 Very Hard computers at once by yourself and win. I normally test random ideas and strategies against 1 Very Hard computer so 4 at the same time would seem to be completely suicidal. The problem is that although I can see a way to turtle up and defend my starting base I can't see how it would be possible to actually push out and capture enough extra bases and territory to defeat the computers. I was considering this problem from the perspective of player vs. player combat though where players would actually look at the screen and draw obvious conclusions instead of just following a simple script.
The trick of course is figuring out where the AI has big holes in its strategies. For example, computers are reasonable at finding and attacking extra bases I make around the map. They see them with a scout and send their big army over to smash them. To counter this I select a map with bases on islands and start new expansions on the islands instead of in normal places. The computers figure out those bases are there but they aren't bright enough to build some flying units and go *smash* me, instead they just build more and more units in their ground army and continue to walk into my impregnable defenses at my main base.
The computers also never learn from past mistakes in a game and just stick to the plan. The first time a person ran an absolutely enormous force into my siege tanks and get ground into hamburger they would decide to try something different next time - maybe use flying units, or maybe just sit outside my base with their completely monstrous army and wait for me to make the first move. The computers can't make those strategy choices though - they get their big army together and their algorithm says attack, so another immense force gets to eat hot tungsten and die. (Starcraft 2 tells me that siege tanks scatter tungsten around when their projectiles explode. Why tungsten, I don't know.)
I figure the most interesting thing is how the strategies and decisions are shifted away from the battle and into the game creation. I can't win against certain challenges on most maps - I spent a while figuring out exactly which map was best to allow me to beat these sorts of achievements. I also had to figure out unit placement, unit mix and costing to sort out exactly how many bases the map would let me safely take so I could be sure to have enough money to actually be able to win at some point and not just survive. Once I figured out the exact strategy I would employ and had a map that was ideal for exploiting the AIs weaknesses the actual play was very easy, which is precisely the opposite of a player vs. player match where the interesting stuff is all in responding to what the player does to respond to you. I guess the real change though is that now that I have beaten 4 Very Hard AIs at once and 2 Insane!!!! AIs at once I can do it again any time I want to, but I know for damn sure just because I beat a player once that I can't do that again with regularity.