> How incompetent can one be at interviewing to need 10 or 20 interviews

Yes, after a full day of interviewing and five interviews, presumably preceded by phone interviews, the people responsible for hiring should be capable of reaching a decision.

I've found that hiring managers who don't value your time lead to managers who don't value your time. They say they want to screen for the best and brightest, but maybe they really want people who are subservient and willing to jump through hoops.

We typically do 5, in one day. 10 sounds like the rare case where the hiring committee was on the fence after those and asked for another interview day.

Well, in my case at the on-site interviews one guy had to ask me from a specific domain (JavaScript/front-end) but asked me questions about system-design, even though other interviewer already did that and they pass a sheet from one another so they don't ask questions from the same category. Not surprisingly, the recruiter called that I have to take another 2 interviews because they were unsure of my front-end skills. To make it worse, one of those 2 interviews was actually re-scheduled because the interviewer simply didn't show up. I did average to ~half of the interviews, for the other ~half I was well above average, for some of them I offered solutions they never thought of and were better in terms of complexity then the one that they had (even though they asked the same question many applicants before, that was an online interview and the recruiter told me that the guy told her that they should definitely hire me). Their decision was simply decided by the fact that the last interview (the one that got re-scheduled for a friday-evening with some guy from USA) who asked me some very, very basic JavaScript questions (such as: creating a form input and showing a message when that input is changed), I was really confused as the questions were so easy and I did not know if he was really expecting the obvious answers. My recruited called me, said that I aced the first (out of the two extra interviews) but the last interviewer said that I'm definitely not fit for the job (did I mention that the guy fell of his chair during the interview, after getting his coffee or something like that, maybe this has influenced his decision).

The interview experience was great, but the outcome really left a sour taste in my mouth, especially after bumping into their poor organizational skills.

TL;DR: Everyone was happy or really happy with my performance during the interviews, except the last guy (who was not even from the same office).