Quick Question: Subframe connectors- WHY didn't they do this from the factory?

American made vehicles all had full frames for many years. Most recent example I've personally seen was 66 Impala. Massive steel frame made of formed members welded together. You would think it was part of a ship. The entire body was lower onto the frame from overhead and bolted on. Lesser margins for error too, especially with a 4 door body. A truck cab and bed are separate so they don't have to be exact.
Unibody was a foreign idea. Much less material and labor involved so daily production count went up and profits went up. Ours lasted as long as they have only because we paid for better roads and highways. They weren't meant to last longer than 10 years anyway, even on smooth roads.
Any 30 year old unibody structure needs some reinforcements today.
I'll add this, If new cars were built today like that 66 Impala was... none of us could afford one.