We equipped our military AND did Lend-Lease. Without L-L we'd have had much less need for war-oriented industry.
Exactly. And created 95 ground force divisions, while building 49,000 M4 Medium Tanks...of which a third or so we gave away as Lend-Lease. So if we wanted to double the number of our own divisions, doesn't that mean that we need to double the 32,000 odd M4 Medium Tanks we kept? Which means we still need the manpower in the factories to build 64,000 instead of 49,000 tanks.
See what I meant? It was a balancing act and the main give and take was manpower versus manufacturing output and unit output. Yes, the US could have done much more, by mobilizing as much as the Germans or Soviets, but if we had it likely would have crippled the postwar economy and thus the worldwide postwar reconstruction.