Because not all two fights are the same. If I'm in a battle against fire elementals my main fire mage with 15 fire skills hotkeyed now use to use other spells she knows. She's had poison arrow since level 5 and I completely forgot she even had it until level 15. There's a myriad of responses you can take to what the enemy is doing which will change what you might use on a particular turn. So yeah, a fight might only last 6 turns, but if I remembered some spells I knew it could have lasted 5. I don't want to use 40 skills in a fight, I want to have my 40 skills on display so if the situation comes up where firefly might be an awesome choice to bottleneck the enemies I don't forget it exists because it's not actively on a skill bar.
I agree on macros, there's no point to them in this game.
