Apparently it's because a lot of agencies use software that automatically scrapes résumés for keywords that match job descriptions and they don't work very well with PDFs.

This isn't a PEBCAK error for once, and that's very surprising because I've learned the hard way that your average recruiter is a professional spammer that will flood your inbox with shitty roles whilst lacking the mental capacity to understand that entry level doesn't mean 5+ years of experience.