I'm sure balance plays a role, but I think a part of it is also just a lack of research. In one of the latest NSB-related videos where HoI4 content designers were interviewed, the lead content designer (Archangel85) said that they mostly look at Wikipedia, which is very surface-level and often can have quite a lot of inaccuracies, as many of us "WW2 buffs" know all too well.
It's also a lot easier to omit historical stockpiles. One could include them for all nations, or even just nations relevant to WW2, and still maintain balance. However, that would also require the implementation of systems to represent the things that historically counteracted immense advantages in quantity. Look at the Eastern Front in summer 1941, for example. When the Germans invaded the USSR, the Russians had about 26,000 tanks, over 75% of them category 1 (brand new) or category 2 (used, but operational), with the great majority of them in the Western Military Districts. The Germans invaded with only about 3,650 tanks, assault guns and tank destroyers, and still absolutely thrashed Russian armour. Air power and particularly AT units of course played a major role as well. In that example, in in-game terms, Russia would need heavy debuffs to simulate their catastrophic ineffectiveness, in spite of their gigantic materiel advantage, in the summer of 1941. And on that note, they'd also need to not only avoid collapse, but then also recover and push the Germans back, all the way to Berlin. Allied lend-lease should play an absolutely vital role in this, but that's already getting into another topic...
Anyway, all this would require much more research and tinkering with the game to get right. Their sandbox approach to HoI4 makes it even harder, by a lot. So I'm not really surprised by the way they've handled this.