The recommendation for serif fonts is standard, although the research on legibility is inconclusive. But pull a book at random from your shelf and open it up: what do you see? Right.
Serif fonts were worse for reading on the blurry screens that we all used to have. But now that print-like resolution on phones and laptops is commonplace, the classic rules for typography have reasserted their relevance on the web. Nevertheless, because of fashion and force of habit, sans-serif fonts still predominate.
The recommendation is not standard. They usually go with their personal biases and recommend sans for websites and serif for print. I can pull plenty of books from my bookshelf that have pages with a little too much ink (and some c's look like o's) or too little ink (and some e's look like c's, d's look like cl's, etc.) For these reasons, most serif fonts are defective even if we don't compare them to sans-serif fonts.
Fashion and force of habit do play a role. And that's why most newsprint websites have reverted to serif fonts. Responsive websites that help to get a job done, like GMail, Protonmail, Yahoo Mail, etc. all sans, whereas NYT, WaPo, LATimes, etc. all serif. I really can't sympathize with a thought-process that motivates them to make their websites look like century-old newspapers. Screenplays are strictly in 12pt Courier. Nothing else will do, not even Courier New. Basically, century-old industries tend to be set in their ways. And the book-printing industry is even older. Small blessings, at least Courier has the benefit of being one of the dyslexic-friendly fonts.
Serif fonts were worse for reading on the blurry screens that we all used to have. But now that print-like resolution on phones and laptops is commonplace, the classic rules for typography have reasserted their relevance on the web. Nevertheless, because of fashion and force of habit, sans-serif fonts still predominate.