This is entirely speculation, but I think it may be because people without degrees are more likely to have chosen the field because they love doing it, and people with degrees are more likely to have chosen the field because it pays well.
During the mid-2000s ZIRP, the tech industry was flush with cash and hurling high six-figure salaries at anyone who could use a flexbox in CSS.
With the promise of all this money and high demand came a massive proliferation of coding bootcamps promising to land you one of these lucrative jobs.
That drew in lots of people with zero genuine interest in computer science.
Anecdotally, everyone in my graduating class majoring in compsci seemed to love the field, but this was back when software dev positions were a lot closer in salary to regular engineering (electrical and mechanical).