Canvas has an easy way of checking if a pdf or other course material is accessible, so many universities are forcing faculty to put all their materials on Canvas. That way if a pdf or powerpoint is not compliant it is immediately flagged. The goal is to reach a "100% accessible" metric.
Note that little of this really helps the students that it is supposed to help, because as you wisely point out, raw HTML is almost by definition extremely accessible. I work in a field that uses Latex and the source code of Latex should also be considered more accessible than the compiled pdf. But for university administrators the only important thing is that the accessibility metric that appears (or used to appear, before today!) on Canvas shows 100% accessible.
Note that little of this really helps the students that it is supposed to help, because as you wisely point out, raw HTML is almost by definition extremely accessible. I work in a field that uses Latex and the source code of Latex should also be considered more accessible than the compiled pdf. But for university administrators the only important thing is that the accessibility metric that appears (or used to appear, before today!) on Canvas shows 100% accessible.