You, sir, are an absolute buffoon. Of all the mechanics you can hate in D&D, you pick the attack of opportunity?>so there was a moment you could've hit me, but didn't because I didn't move...cool.
When you're fighting an opponent, you face him and keep your guard up. If you use your turn to attack, you don't leave them an opening, as you're being active and alert to attack him, press on him, and prevent him from counter-attacking. They can't attack you because you're being fighting them actively and not leaving them any opening. If you turn heels on them, or spend a solid six second on chanting spells or reloading crossbows or pulling your bow and shooting at someone else, then you leave yourself completely open to them and allow them to capitalize on that moment to strike. It works perfectly both in terms of logic, turn economy, and game depth/balancement.
I don't play D&D, but I borrow aoo in my games because it just works well.
Be ashamed of how much of an idiot you've made of yourself today, OP.