HR do the recruiting, and have to fit the recruit into a company-wide HR policy pay-and-benefits grading structure. That structure takes account of education.
HR also know nothing about tech (and are usually bizarrely proud of it). So they need the tech people recruiting to lay out a range of specific qualifications or experience that the recruit needs, so they can easily reduce the huge pile of CVs down to something more manageable.
The combination results in a degree being required, and CV's with no degree never even make it to a review stage.
It sucks, because often the best programmers are not degree-qualified. But large organisations aren't actually a meritocracy, or concerned with hiring the best people. They want the people who will fit into the large organisation and its bureaucracy nicely. A degree usually indicates at least some respect for authority, so actually it's a good qualifier.
This is so true in every industry. My company did work for a big HR firm that specialized in servicing the accounting industry. Every one working for that firm was aggressively ignorant about every aspect of accounting. Having known people in other HR fields, that tends to be true overall. It almost feels like they think that finding out more about the industry they are servicing would somehow lower them to the level of the "peons" they are hiring. This is anecdotal only, of course.
I've worked with very good programers that don't have a degree. But, learning the basis to become a good software engineer, is just a lot easier following formal training (like going to University). I find it hard to believe that most competent programmers will not have a degree.
Again, having a degree is not necessary to be a good software engineer. But it's a lot easier to be a good software engineer if you have a degree.
When hired, I was told that my job is to prepare the students to work in a cubicle.
This is reflected in the curriculum - for example, proprietary software is taught (even where open source tools excel), because they are more likely to be found in the large corporation they are being prepared for.
If you think of cubicle training as the main job of the university, it follows that large corporations would look for degrees.
If you read The History of Education (1920)[0] which outlines how the American and the Preussian system of education it is based on was created and on what basis, you'll see that this is exactly true. Education has never, ever been about exploration, or knowledge for its own sake. It has always been about increasing productivity for businesses, and tax revenue for government.
[0] https://archive.org/stream/briefhistoryofed00cubbuoft/briefh...
For certain departments an amount of prestige is owed not just to the research you put out, but how many students you successfully place in jobs in the field.
I studied geography at a mid tier public school and this school of thought was pretty prevalent. The professors used open source software in their research but when I learned statistics it was IBM SPSS and GIS was all ESRI and MapInfo. The department even hand curates a list of alumni and their current employers for marketability to future students.
Recruiters basically keep a database of engineers and contact them to get them into the typical interview funnel. Interviews are done by engineers and their managers only and then everyone gets together in a meeting soon after to give a hire/no hire. Then recruiters (and managers) attempt to close the interviewee. This has been my experience after interviewing at dozens of companies at various sizes, from small to huge.