> While I agree, how much training does anyone get as an interviewer?

TL;DR: not enough training.

As a hiring manager, whenever we start a hiring period I have a conversation with my interviewer team about what qualities we're looking for and review the questions they plan to ask in order to normalize the process. Stuff like "what does a good answer look like, and why? what does a bad answer look like? is this something easy for a candidate to engage with or will you spend half the interview just explaining the background? is this coding question unreasonably hard?" and so on.

I also look at the evaluations that interviewers give relative to other interviewers. Almost every hiring cycle I've done I've had to deal with some interviewer (all over the seniority spectrum) asking unreasonably hard questions.