The problem is that NATO has entrenched their manufacturing and industry to fight a war according to their doctrine, which is heavy on high-tech air support, and not so much on conventional artillery. Ukraine, even with the small amounts of western equipment they have, has the capability to fire over 350,000 155mm shells per month. The US currently has the capacity to produce a measly 15k/mo, and all of the EU can only do something like 50-75k/mo, IIRC. We just can't keep up with that rate of expenditure. Imagine how many more we'd go through if NATO was in an actual war with all of it's equipment.
At least we seem to have learned a lesson from it (masses of artillery are really important in a war of attrition), and the US is now increasing production by something like 5x (but that's still a far cry from what you'd want to be expending in a war). We have the high-tech manufacturing capability, but it's slow and expensive. We also need to maintain the industrial capacity to churn out dumb, cheap, lots. It would probably be way more difficult today to turn an automotive assembly line into one for aircraft, like we did during WWII.