There are two ideas I'm trying to reconcile - that "if you've gotten this far [the interview] you're good enough to be here", and the fact that only about 30-40% of interviewees will get an acceptance (at least at the schools where I've interviewed).
So I'm really curious - what is the distribution of interviewee performance from "really bad" to "truly exceptional"? I imagine most fall in a nebulous heap in the middle and are not a distinct "yea" or "nay". How on earth does an ADCOM get enough information out of the interview process to tease out that top 30%?
Or is it really the case that 50% of otherwise highly qualified people suck at interviewing?
I'm not an adcom, but I am pretty sure that after studying this for two years I have some reasonable inkling of what's going on. IMHO, you are just not looking at it correctly. Getting an II doesn't mean anything more than you are good enough to make it to the next stage of the process. It doesn't necessarily mean you're good enough to be admitted.
The 15-75% post-II admit rate (it varies by school and the spread really is that large) is by design, not accident. Schools invite a lot of people in because they have capacity to do so, and want to have a lot of choices. My favorite explanation for how interview performance factors into a final decision is
@LizzyM's staircase analogy.
Under that explanation, it's not that everyone with an II is "good enough to be here" and everyone starts with a clean slate, with the top 30% of interviewees receiving As. Instead, people are ranked going into the interview, and the interview is just one more metric used to make decisions. Therefore, you can easily be at the bottom of the staircase going into the interview, have a great interview, and still not be accepted.
Ultimately, it has nothing to do with a ton of people sucking at interviewing. Sure, some do, but the bigger issue is that all schools choose to interview far more people than they can admit, so a lot of people are going to be disappointed. It's pointless to look for a deeper explanation than that, because one doesn't exist.
Yes, someone with 10 IIs and no As is either really unlucky or just sucks at interviewing, but that's very rare, and doesn't account for the fact that the stats suggest that you should expect 1 A for every 3 or so IIs. It's just the way the schools have set the process up, and has nothing to do with adcoms teasing out the top 30% based primarily on interview performance.