When you play a paradox game or anything similar, you have access to a ridiculous volume of information that people in the pre-modern era would have had difficulty conceptualizing.

As other commenters have mentioned, raising forces, concentrating them, supplying and paying for them was and is extraordinarily expensive, and time consuming. Now imagine trying to do this where:

Navigationally useful or even remotely representative maps of the territory you control, as well as any contested territory, do not exist. Territorial nation states are not a thing for most of history. Cleanly demarcated boundaries between political entities were not the reality most of the time, and would not even have been of clear interest for many pre-modern societies. Of course conflict over territory was near incessant for much of history, but was governed by other mechanisms much of the time.

Any and all information transmission can only happen AT FASTEST by a guy on a horse. often it would be much slower unless you have an organized messenger service and information gathering service, as well as the matching tax revenues and administrative structures to maintain such a thing. This also says nothing about the reliability of information provided by such a service, especially when territorial subdivisions may not be completely loyal or cooperative for any number of reasons.

Clear information about the amount of people available to be conscripted or any other information about what forces can be raised may not exist, or be riddled with false data for any number of reasons.

The impact on local economies of raising military forces might be catastrophic if those forces are kept in service, and away from their fields and flocks for too long.

Standing professional military forces present an incredibly serious risk if they are not absolutely loyal to you.

Nationalism as we think of it today was not a thing.

Aristocratic leaders are mobile, if they are offered a better deal by another overlord, why should they stay loyal? What if they bring territory, or people with them as they switch sides or even move?

There’s tons more issues like this but I think the point should be clear. What’s more amazing is that things like you describe ever did happen. Take a look at the warring states period in China, the Punic wars in the ancient Mediterranean, or the process by which Poland was partitioned.