[PeterisP]

The traditional answer to this comes from the age when college graduates were a distinct minority. In the 1950s environment, if a job applicant has a college degree even with irrelevant subject, then this means that the applicant has passed two filters (getting into college, and actually graduating with decent grades) for qualities that are relevant to most jobs but are hard to measure directly, so that degree is a very useful signal for evaluating people in a way that you can't do during an interview or two.

These qualities are not guaranteed but are correlated with being able to get into college and graduate - everything from conscientiousness, ability to follow arbitrary complex rules, general intelligence and also socioeconomic status (in e.g. sales and management, the social contacts of a high-SES employee and their family are very valuable in achieving business goals, and the social contacts of a low-SES employee are not). A graduate might not have these qualities, and a non-graduate might have them, but it's hard to tell so the degree is a useful proxy because it does (or did?) correlate with these qualities.

So it made all sense for businesses to prefer college graduates for certain types of jobs ("the good jobs") even if the college major was something like history or literature in a field of business where that's not relevant, and they did just that. And because of that employer preference, a college degree was a ticket to one of these good jobs.

However, if almost everyone gets a degree that means that there's no real filtering happening, and that benefit gradually becomes useless.