Chaosmancer
Legend
I'm not sure it is a problem. If I were reading a novel, most novels at least, little world building tidbits like that kind of nag at me. Because you're absolutely right, it takes a tremendous amount of resources to train, equip, and maintain a significant fighting force and that will rely on some very sophisticated infrastructure (even when taking into account magic and whatnot).
But I admit that I take a slightly different approach to world building for a game. I typically only concern myself with what matters during the actual game play. Does it matter how my city of a million people feeds itself? Not unless it matters during the adventure. Does it matter how a seemingly "tribal" society with no cities, countries, or empires of their own maintain a well trained and equipped standing army? Not unless it matters during the adventure.
And it gets a little odder still, because while I accept that for fantasy games like D&D, I don't necessarily accept that kind of thinking in other games. I've a tough time figuring out how a wooden suburbian home in what was once the United State s is still standing after 200 years of neglect or how Yum Yum Deviled Eggs can remain in their package for the same amount of time and be edible. I just sing to myself:
If you're wondering how he eats and breathes/
And other science facts/
Tra la la/
Just repeat to yourself/
It's just a show/
I should really just relax
But my way is not the only way. I recogize the validity of your approach to world building and recognize that it leads to a richer more interesting setting.
Right, the issue though is that we don't just have the game. We have the setting. We have the maps and the numbers and when we see that a place the size of a medieval city has a population of a modern super-urban center, we start realizing that the set dressing is fake. And that means the world doesn't draw us into considering the problems it presents us with.
And if you have the orc lands, the giant lands, the dragon lands, and all these places mapped out, then you have a massive hobgoblin army appear out of nowhere, you may be able to get your players to ignore it... until you give them a name. Until you say "this is the army of [Blank] led by [Blank]", which you need to do. You need this army to be personalized, it needs leaders, it needs goals and directives, especially if it is a major antangonist... and that leads to "wait, where did they come from?" and "Why didn't anyone notice this problem sooner?" And you can say "from the hobgoblin lands" but when the players ask "where is that on the map you gave us?" then you are in deep trouble. Because it isn't on the map. And the Realms is particularly vulnerable/bad about this, because the Realms is highly detailed, with little left to the imagination... and no hobgoblin kingdoms. No place where they have ACTUALLY conquered the land and taken over.
And again, this may not matter during actual play... unless you players want to do literally anything about this threat other than try and kill every hobgoblin in the army. You can't negotiate with them if they have no lands, because then they have no wants or needs, you can't find allies that aren't already established, because no one else is threatened by them by having a border with them. Everything suddenly becomes much harder for the players, because all of this information is just... non-existent