It's inflation IMO. Wages started stagnating in the 70s which is exactly when the USD became completely unbacked (due to the end of Bretton Woods), enabling the government to go endlessly deep into debt, which we proceeded to do with gusto, sending inflation skyrocketing.

Somebody who's earning 20% more today than they were 5 years ago would probably think they're on, at least, a reasonable career trajectory. In reality they would be earning less in real terms than they were 5 years ago, thanks to inflation.

In times of low or no inflation it's impossible for this happen. But with inflation it becomes very difficult for workers to really appreciate how much they're earning, and it enables employers to even cut wages while their employees smile about receiving a 2% 'pay raise' when they should be raging about the pay cut they just took.