I think the issue is that it's basically making two, three, or even four games in one. And nobody is going to that much extra for it, and if anything it hurts sales because instead of selling four different games you're selling one really hard to make one.
The economics aren't there. Hoi could have done the ww1 and ww2 era is they really tried for it, while Victoria 2 would be able to shift a few years earlier, and then a new game could run the cold war (probably looking a lot like victoria tbh).
A modern day game probably won't sell well, since it's either going to be boring or unrealistic.
I completely agree.
Though that said, the problem with putting multiple eras in a single game is that it will inevitably become very generic, due to its very nature of having to use streamlined mechanisms that can work for every age it depicts.
This works with Civ series because it is immensely enjoyable whether focusing on a single time period, or spanning from bronze age to space era. It has a strong foundation that cannot be shaken, and it has consistent mechanisms. And can take huge liberties since it is only based on history to an extent.
But a Paradox game (or for that matter, a Total War game for example) can only handle a single time period (or transition from one age to another) at best, and other timeframes suffer in one way or another. Any attempts allow playing them all in individual detail would almost push the limits of computer specification.
Take CK2 for example, it can barely depict the early medieval era (the Dark Age) right. EU4's mechanisms shine best around 1500s to 1700, but are inaccurate for anything before or after that timespan (professional standing armies in 1444 for example). Victoria 2 can depict early and mid-industrial revolution around the globe better than any other game in history of gaming, but almost falls flat as soon as the modern industrialized societies appear in 1890s and has to use workarounds.
Or for Total War examples, Shogun 2 is the only game that depicts transitions well - from feudal armies of peasants, archers and Samurai into very professional forces of disciplined, gun-armed soldiers...and then from that into late 19th century modern industrialized society and military. Medieval 2 Total War (from early medieval to late), and to an extent Attila (transition from huge urban empires to collection of semi-tribal kingdoms) and Empire (essentially EU4 - small medieval-esque kingdoms and rich empires to large modernized colonial powers). Each of those game had to sacrifice element of one era to depict the transition or multiple of them.
So yes, a game spanning 1821 to say 2000s is definitely possible. But it would either have to incorporate so many mechanisms and emerging features that it would become expensive and heavy on the computer...or it would have to be blanded down to something generic enough to handle everything yet not immerse or satisfy lovers of any era perfectly.
