Why should we have 5 generations of 20th/21st century weapons tech, when we have only included 5 generations of pre-20th century weapons tech (ancient, classical, medieval, renaissance, napoleonic)?
That kind of modern detail is better suited for mods than the main game.
How much difference is there really between 1980s "cold war" tech and "modern" tech? With a handful of exceptions (laser-guided bombs, drones) the tech is basically the same - particularly outside the US army.
Because that isn't most of human history.