This question already has an answer here:
Following up with this question Has there ever been a strike of peer reviewers? and this Why don't researchers request payment for refereeing?
I understand that peer reviewing probably started when there were relatively fewer article submissions than today and when the academic system was going at a slower pace and was less cluttered with people.
But given how the system has evolved, I am wondering why do employers let their employee work for free for the monetary benefit of a third party? As far I as know no employer pays its employees to make someone else rich. It seems against any economics principle. Wouldn't it be more efficient to pay reviewers so they would be more motivated, quicker to review, and universities/grants/governments would not pay someone to work for free for the economic benefit of someone else? This might also offset the cost of salary of professors. Any economic return to the employer seems very low considering the working hours devoted to reviewing.