Part of the problem is the sheer scale of hiring, but I think most of the problem comes down to the lack of feedback or evaluation mechanisms on the interviewing side.
They train you up, half a days worth, training tells you not to be an arsehole, not to ask stupid questions, not to have unreasonable demands of candidates, not to be biased. They don't train you in valuable skills like active listening.
Next thing you know, you've done training, and you're interviewing candidates every week or two (or more often), and there is zero feedback mechanism. No one evaluates your interview questions, no one asks candidates to provide feedback on the interviewers. No one looks to see if you've got unreasonable expectations as an interviewer, or have your expectations set too low.
You do have to do a post-interview group discussion with the other interviewers to make a yay/nay decision, but it's super easy to present what you did/asked in a positive light.
The whole system is designed around the interviewer being right and infallible. Is it any wonder the process is so completely and utterly broken?
edit: > or have your expectations set too low
This is where I bias towards in worrying, imposter syndrome and all that jazz. I've made a conscious choice to not raise that bar higher. I think the questions I ask are good, I think they're set up well enough to encourage candidates to go as deep as they feel comfortable with. I try to design them with no one true answer but have a few in mind so I can go where the candidate goes.