OP's co-author here.

The biggest difference is that internships are short and low-stakes, so if something goes off the rails it isn't too costly for the managee or manager. Also, at least in tech, hiring is hard enough that interns tend to get the benefit of the doubt on "performance issues" in my experience.

I can think of a few things. Interns are even less experienced than junior engineers, so they might not know when they're in a bad situation. They also have a pretty decent incentive to not make waves, since they're typically being graded on their work, and evaluated for the possibility of a return offer.

On the other side of the equation, when I mentored an intern at my company, I was given a great deal of support and training in how to help them have a successful internship. This may or may not be the case for a new manager.

Anecdote, but when I was an intern back in college I had a single designated mentor but his job mostly consisted of getting me to be more confident and ask fewer questions when I could spend more time researching solutions myself.

My code was also subject to in-person code reviews by several engineers which was extremely valuable as it made me more comfortable with the whole team. This allowed me to become comfortable enough to ask other members of the team questions, freeing my mentor from being overwhelmed by me.

It was very much a "takes a village" approach and was an extremely valuable experience. Again, though, there was also a lot of emphasis on not wasting too much of any one person's time. Mostly because the team was a bit too happy to help me out for several hours instead of doing their own (much higher priority) work.

I think it's because being an intern is not as bad for a person's ego. Like if you have a manager and a report they are basically the same, except one is above and one is below. "Why me below?" Well in the intern case that question has a very obvious answer. Plus you know it's for a limited time.