That was efficient and a good choice for the situation. Against a peer foe like the Soviet Union, you need lots of extra capacity to supply needs during a war when everything is consumed quickly, from ships to bullets. From the end of the Cold War until recently, that was unnecessary. The Taliban, Iraq, Serbia, Libya, etc. were not going to require so many munitions that there would be a crisis; they weren't going to sink any destroyers or shoot down any planes.
China is a 'near-peer' threat. As Russia's invasion of Ukraine has demonstrated, the capability of all of NATO - with combined economies maybe 30x Russia's - to manufacture supplies for Ukraine is insufficient. A war with China could be much larger; the US needs far more capacity.
Another, more critical reason is that some weaknesses are escalatory - they encourage escalation by the enemy. Chinese leaders, seeing the US lacks that capacity, are incentivised to escalate peace to conflict, conflict to war, a small war to a large war. Escalation is the bane of international relations - it is systemic and often cannot be controlled, leading to catastrophes that nobody wants but nobody can stop. Any professional foreign policy carefully constructs situations that are systemically de-escalatory.
(As an example, the US Space Force is changing from using a few expensive super-satellites to using very many cheap ones, in large part because having a few super-satellites is escalatory - it incentivizes an enemy to conduct a surprise attack, because they can knock out a large amount of capability at once.)
[0] OT: also a mathematics Ph.D. Do such people still rise to leadership in government? Or are they all maximizing click-thru for SV companies?