Well, DUHHHH!
It's kind of the feature of the whole thing that everyone is missing. One side dug in deep using NATO designed fortifications and WWI strategy, thinking the other side couldn't do anything. The other side, it turns out, had a s4it-ton of artilery, ammunition and lots of old decrepit backwards factories that could churn out a lot more of the same. Cheaply.
The US blundered into the kind of attrition war we swore we were never ever ever going to fight, having learned the lesson of WWI so well we never had to remember it.