STEM has the same issues as humanities when it comes to academia, but the difference is that for graduate students, there's often (although not always) a straighter path into industry.

I’ve noticed that, but I think it hit the humanities in the 1980s and arrived at STEM more recently. It’s just the MBA-driven financialization and enshittification of everything.

But it’s ultimately down to the fact that a college degree is no longer a ticket to the middle class, so it matters a lot what degree and from which school.

It was a fundamental confusion of cause and effect. People noted that the college educated earned more, so assumed that expanding college will confer that same status to all that obtain that pledge. But it inflated its value. Similarly if you squeeze everyone through high school and look the other way even if they don't match the criteria, you just inflate the value of the high school diploma instead of giving the previous high school graduate prestige to everyone. Then the same happened with undergrad. More students, less requirements and then surprise that you don't get an automatic college wage premium for having studied English literature or psychology or communication at some low tier college.

Some people just don't seem to understand that the value of a credential rests entirely on the fact that other people don't have it.

In part, but that wouldn't be enough. The value is that only/mostly people with certain skills and talents have it.

Partially. If everyone can read and write, this doesn't make a certificate of being able to read and write meaningless. Well, it makes the certificate meaningless. But the skill is still there and it's good that everyone has it.