They no longer believe it is worthwhile, because the landscape changed: companies found they no longer needed to treat their employees was well as they had. (Driven largely by the shift around that time toward quarterly results over long term sustainability, as I understand it.) And thus began a race to the bottom.
Globalization played (plays) a huge part in this timeline too. If I can outsource your job to a place that really does treat their employees as disposable, you should just be happy to be employed. And even if I don’t want to offshore, how does my product/service succeed or remain sustainable if my competitors are all offshoring (or indeed come from offshore)? I’m not defending this attitude just distilling and illustrating through extreme language what I see as a reality of global competition.
Post-WW2, America had a lock on global manufacturing and was like last man standing in a burned down world.
It was/is an illusion to think that could be a permanent state.
The pre-war employment situation in America looked nothing like post-war. Your race to the bottom narrative is probably better framed as reversion to a multi-polar world with bonus features of higher global prosperity and capability, lower barriers to access foreign markets (whether laborers or consumers), and mature logistics infrastructure. In short - more people than ever want YOUR job, and to live in YOUR house, and have YOUR safety net, such as it is, it’s not just some focus on quarterly reports.