I want to know if there was some sort of HR training that made that language more common versus just neutral “we have a job posting”

Having been on searches and in search depts, "Join us!" is indeed pretty much all that HR wants us to say-- they get very twitchy about anything that isn't the Officially Approved Ad, in ways that get totally ludicrous. (1)

strong hunch from me is this is bog-standard "academics suck at theory of mind, on average" the phrase does literally nothing to increase the likelihood of people applying, and makes them miss an opportunity to actually say something useful