AOO and several other reaction spells are already in the game and it happens automatically that's good enough.
It really isn't though. Right now the reactions don't even always work or happen, but even if they were to work; you don't always know what you want to do until the battle is happening, and then you might have clicked away some choices you truly wanted to use, but even before they happen, you might have already procced a reaction that you didn't want, like an AOO, but instead it turned out you should have saved your reaction for a shield spell.
But even if you only have the shield spell (for instance) active, what about a situation where a small little critter comes up and hits you, and you wouldn't want to waste a spell slot and a reaction on that, because a far larger threat will attack you afterwards? I know the shield spell lasts the round out, but it could be some other reaction. In the cases of reactions, it's all about prioritising depending on what the enemies do and what you think is the best choice for the time being, not depending on automatic procs.