To play devil's advocate, it's a slippery slope. If you introduce flatfooted AC, there's a whole bunch of places it makes sense that you need to balance. You get questions like "well, we surprised our foes: do we have advantage, are we attacking their flatfooted AC, or both?"
Advantage is not supposed to be perfectly accurate in terms of what it emulates. It's supposed to be quick and do a good-enough job of emulating. There are always "yes but" that we can come up with. "Both my PC and that ogre are paralyzed and have AC 14 - I'm a smaller target, why am I hit as easily?" The collective We of D&D players could come up with a ridiculous list of reasons and specific modifiers. And we did in some editions. Look up half standing in water for 3.5, where there's cover but also the water has another effect.
So we have something that is "good enough" in terms of accuracy, and absolutely tops in terms of speed of resolution and simplicity. Plus players like to roll more dice. That's a hallmark of 5e. I really like other editions of D&D as well, and a DMign style that wants that level of realism may be better suited to another one of my favorites, 3.5. Use the tools that fit your DMing or playing style.