It was made so complex already that the added complexity of having three armor classes would have been lost within. Alternity had three armor classes, but Alternity wasn't as complex as Starfinder. Alternity had High Impact (HI), Low Impact, (LI), and Energy Weapons (En) The only difference is Alternity Armor class used counter dice to absorb points of damage after it was resolved whether an attack hit or missed, if it absorbed all of the damage then it might as well have been a miss even if a hit was scored. But Alternity did have three kinds of armor classes.
Now why is realism important? Because if it is not important, then players would not fight like modern soldiers, they would just stand there in a shiny suit of armor taking hits while running up to the enemy and slashing him with a sword. Not exactly how a modern soldier would fight.