I think unsaid about a version of that belief is "... and, after expensive ramp-up including mentoring, they'll probably job-hop for a better salary, or better company, before the company sees payoff."
It's not just latency on payoff, but risk of any payoff at all.
> “...interns bring [...]"
"...a relatively low-cost way to evaluate a candidate hire, and then, if they show promise, you have their attention to make them an offer for a real job."
Given how hard tech companies find vetting candidates (e.g., many still cling to Leetcode and what school someone went to), and the demands of post-ZIRP on effectiveness, interns (and expecting to spend more on mentoring than the interns produce in value) are one solution for finding good future junior hires.