Part of this might be the increase in the use of high-tech, and therefore expensive and hard to produce, weapon systems.
Earlier wars could be supported by a “war effort” in which factories that used to supply consumer items were re-purposed to produce tanks and the like. The US was famous for producing a plane every X minutes or so during the height of WWII.
But even the US can’t do that when the planes are F-35s.
So now nations go to war “With the army they have”, to borrow a phrase from the start of the Iraq war. The expectation that battle losses will be quickly replaced, and that production will ramp up so as to exceed battle loses no longer holds. You’d expect a few new planes and tanks every year, but if you’re losing hundreds, you’re never going to make up for that level of losses.
And so you husband your resources more closely. You only commit forces that you’re pretty sure you aren’t going to lose immediately. You also make more use out of things like missiles that can be used from a distance.
Russia thought it was doing that in Ukraine, but their expectations were not in line with reality.