Until quite recently (post the 1970s in most universities) , professors were permitted to write ---- but not pushed to write.
But starting in the 1870s and increasingly by the 1920s, amateurs were pushed out of all the learned societies and the assumption gradually became that only PhDs in a few rich big universities in a few world class big cities could do serious scholarly work.
Others - amateur authors and people with advanced degrees working in business, government or teaching in minor universities or high schools - could provide useful primary material (write a novel , pass an Act or invent a machine) but weren't ever to be cited for secondary insights.
Research university professors should only cite other research university professors for intellectual substance: a nice tight profitable closed loop of about a dozen research universities in each of the biggest academic nations, maybe only two or three in smaller developed nations.
One natural response was that all universities and colleges, particularly those out in the boonies , increasingly pushed all their professors to also publish or perish, once they had a bit of money to throw around to help make it happen.
This was a half good move ---- in that indeed good ideas can come from anywhere - but why then seek them only from people with PhDs and tenure ?
It happened because traditionally it is the author's job to have ideas and the pure teacher's job to try to control and assess them , less young minds be misled in ways that threaten the powerful.
We speak of the profession of teacher and presume it requires a long education and the passing of rigorous exams - while even today , anyone is permitted to write books* and have ideas - no education certificates required.
*Actually commercial realities became the new gatekeeper - an author could self-publish anything but if it wasn't published by a cost and profit conscious commercial or university press, all the (university trained) newspaper reviewers agreed to dismiss it as worthless.
But by the 1840s , ideas no longer only went public via university teachers filtering them before teaching some of them to future school masters and parsons.
Now authorship (amateur , untutored genius) could reach its potential audience directly - unmediated by the university gatekeepers - due to the growth in cheap fast steam printing presses together with the new railways, steamships, telegraphs and the public mail to distribute them widely.
That won't do --- ideas are dangerous - too dangerous to be left uncontrolled in the hands of the unwashed democracy of amateur authors.
Ideas - important ideas at least - now only come from PhDs with tenure at well regarded universities, have been peer-reviewed, perhaps at the journal article level, and are published by well regarded university or commercial publishers in a hardcover edition.
That's a lot of gatekeepers - enough to safely dilute any dangerous idea , wouldn't you say ?