Nothing to do with AI
Junior employees have a higher level of educational attainment because today you can't get a career without a degree. Companies today require a degree for junior positions that in the past a good academic achievement at school (O Levels) or sixform (A levels) would have been enough. Because not that many people entering the job market for the first time had degrees. As there were nowhere near the number of degree places, universities or shear variety of degree subjects.
When I left school in the 1970s very few people went on to degrees, to get say a job in a drawing office (draftsman) or a career in retail management good O Levels or wait 2 year for good A Levels would have got you in. Then it became A levels and today it will be vocational degree in drafting or retail management.
AI or not, juniors are going have degrees because without them they stand no chance of getting the first job of a career.
As for the "with greater emphasis on STEM degrees", I wonder what qualifies as STEM are they including social sciences as they do in the US or are they excluding them as we do in the UK.