Because that's what those particular games are all about. Starcraft is about dynamically selecting a strategy and manually executing it via lots of busy work. To make the game "easy to learn but impossible to master" they've made the unit AI deliberately dumb, so that beginners can easily pick it up and play, but also so that a "masterful" player can improve the outcomes of battles by manually overriding the AI -- e.g. pulling back wounded units, kiting slow units, targetting enemies in optimal order, etc -- and certain abilities have no AI at all, e.g. force-fielding a ramp requires a perfectly timed and placed click. These are just little "mini-games" that you can master -- e.g. banelings rolling at your marines, then it's time to play the "loose formation" mini-game.
If micro management was so bad..
then why doesnt rts games like the ones i mentioned make units automatically start kiting if they get attacked and do everything automatically so rts players dont have to micro manage their units anymore...
They've also used multi-tasking of "stupid robot work" to differentiate between beginners and masters -- e.g. remembering to select your barracks continually and keep your queues full of marines, or remembering to select your queen every 45s to inject your hatchery with larvae -- these things are just extremely simple "mini-games" that they're forcing you to play at the same time.
They could've provided AI or buttons for most of these things -- a button to keep building marines until you say to stop, a button for queens to auto-inject, a "spread out" button to minimize baneling splash -- and other games do provide these kinds of features -- but then the game would've been 100% focussed on strategy, instead of being a combination of strategy and unit control skills.
Starcraft is all about these mini-games -- many brood war fans have even complained about the "dumbing down" of SC2 by allowing the user to select more than 12 units at once... They've deliberately struck a balance between AI control and manual control to make the game easy to pick up, but impossible to become a true master of. Even when watching guys that get paid $300,000/year to play the game professionally, they still make mistakes in micro, macro and overall strategy.
This isn't the right design choice for every game. Some games would be more fun with better default AI, and not every game needs to provide as many opportunities to differentiate skill levels as an "e-sport grade" RTS like starcraft has to.
e.g. a lot of people enjoy a nice casual game of chess, but it's a small minority of those people that also enjoy intense speed chess, or the competetive chess tournament scene. Most people would probably think that chess would be ruined by introducing combat mini-games where pawns with "good micro" can start beating queens...
As a different example, I used to play a lot of counter-strike -- I had pretty good skills in map control and predicting the behaviour of other players, which meant that I could often "get the drop on" other players so I would be in an advantageous position when the firefight began... however, my aiming skills weren't the best. If your opponent has better skill at getting headshots (or has an aimbot), then strategy doesn't matter; you shoot them in the back 4 times when your MP5, and they 180 noscope you in the head and win the fight despite playing recklessly... That game is all about the "micro" of aiming skills, and strategy only comes into effect when both teams are equal in aiming skills.
Compare this with the original Rainbow 6 game, where every player had auto-aim enabled -- your crosshair would automatically move to the forehead of any enemy on-screen! This had the effect of making the "micro" of aiming irrelevant, and making the skill test all about strategy, positioning, and the timing of your plans. If both teams were of equal skill, then perhaps their reaction speeds would then come into effect to differentiate the two.
You can think of these games as simply being sorting algorithms of players -- CS sorts first by reflexes, and then by positioning, R6 sorts first by positioning, then by reflexes.
Two different rule sets, two different prioritisation of skill sets in the sorting of winners vs losers. People who don't care about honing their reflexes or practising the skill of using a mouse to click on small targets, but do enjoy the strategy of firefights, will probably have more fun with the latter game. People who do enjoy clicking on small things quicker than other people, will probably have more fun with the former game.
So back to the original question, are you designing an e-sport (like Starcraft), or a casual hobby (like WoW)?