In short, no matter how realistic the weapons, aiming mechanics, movement mechanics, and environment "set pieces" are in tactical shooters, they all have one huge gaping hole in realism; you play on the same confined "battle arenas" wave after wave, round after round, match after match, month after month, year after year.
This simple factor causes probably all of the absolute most unrealistic tactics and behavior in tactical shooters. It's not just about knowing the lay of the land, it's the fact that everyone starts in the same places at the same time every round. Thus, all combat in the game turns into a routine where people take the same paths and run through the same chokepoints and danger zones at the same time every time you play.
One thing which is just waiting to be done in tactical shooters, but hasn't yet is procedurally-generated maps. Procedurally-generated content has actually been around since the early days of gaming, when there wasn't enough memory to support large quantities of pre-made content. Based on certain algorithms and parameters, the game would essentially create graphics, or characters, or levels on the fly.