As Kylotan said. The scale of warfare is shrinking and becoming more asymmetric in the modern age. Which is a good thing, because several powerful nations duking it out with modern armies would probably cause immense destruction. WWII leveled a lot of cities. It makes for impressive gaming and impressive history, but it was a nightmare to live through when it was rolling through your back yard.

That being said, the concept of cleanly discrete battles started to lose meaning in the modern age of warfare. You didn't just have two armies meeting and driving each other off a battlefield in a couple of hours. During both world wars and every since they had Battles with a capital B to describe major actions, but the actual fighting was constant. Raids would go back and forth between battle lines on a daily basis, small groups of soldiers trying to maneuver would cause firefights and occasionally larger engagements to break out unplanned. How much of this "small" action you put into the hands of the player is a tricky question.

but why no modern-themed games like total war?

i'm guessing two basic reasons

1. modern war is sufficiently dissimilar to the capabilities of the total war engine that The Creative Assembly has decided to stick with the types of game settings (locations, time periods, and unit types) that the engine can already handle. That's my guess as to why The Creative Assembly hasn't done it.

2. The Total War engine is an absolute tour-de-force when it comes to amazing LOD capabilities and large numbers of entities active at once - like 5000 of them! Definitely not a trivial undertaking. I suspect this is why we don't see anyone else doing it - especially combined with the fact that wargames are niche, not mass market products.

The scaling and time factor is a very difficult part of doing modern warfare modelling if you want overarching strategy in the same game that gives you low level tactical control.

Singular battles with well defined boundaries and durations aren't really 'a thing' anymore in large scale modern warfare. Where is the 'edge' of the battle? The artillery pieces miles off offering support? The opposing battery firing on your support artillery from even farther away? An airfield launching support craft? What about the military base on the other side of the globe currently loading transport aircraft with special forces paratroopers that are inbound to help?

If you have tactical control over a specific battle along with strategic control over the entire war, when what happens when you send a column of troops from one strategic location to the battle you're currently in tactical control of, and they get ambushed? Do you take tactical control there as well? Do you give it to the computer?

In short, if you're pushing things beyond the era of soldiers lining up to do volley fire across fields, then you're getting into the point of warfare becoming far too complex and fluid to allow a single person to keep even the remotest of grasps over the entire thing. It becomes too difficult to section it into manageable pieces that allow control and decisions to be made at all levels in a reasonable manner.

I would strongly suggest that anyone looking at modern military actions for a game setting to pick a military rank, and base a game around roughly what they actually control in the real world, and don't let the game expand beyond that focus. Playing as the commander of a platoon? Move squads around and change their formations. Commander of a company? Move platoons around and either abstract away the lower level movements or design an AI good enough to simulate someone else controlling them.

The higher up the command line you go, the larger the map scope and the more vague the fine details. If you're overseeing an entire global war, then you're not really all that worried about a specific pillbox or something somewhere in the world. You're worried that the front in the region is still advancing, and whether or not you're going to spend resources to reinforce it or not.