Another way to look at it is you get lots of prestige by leading the field, and you lead the field by teaching people at conference presentations. Admittedly the people being taught are fellow academics not freshmen, but still...
Finally you could survive in a tenured job with no motivation to be leaders, but given the overproduction of PHDs ranges from merely 10x to as much as 1000x as many as necessary in some liberal arts, the odds of a slacker being hired when only the top 1% can get a tenure track prof job are extremely low, so there is that selection bias against slackers indicating very few profs will be slackers.