Cookiecaper, I understand where you are coming from but complaining only makes you look like a sore loser. You are not helping your position by this post. Consider taking constructive action.
Generally, we've found it best not to look away from the mistakes we make, and we make our fair share of them! By doing that we hope to not repeat them, or at least repeat them less often.
There are many valid and plausible reasons that they could've chosen not to move forward in the interview process: maybe they got to another hireable candidate before they even seriously considered my application, maybe there is some specific technical need that I clearly don't meet and which I'm not picking up on from the job description, maybe something else.
And I can't see anyone else in the candidate pool either; while I'm good at things, undoubtedly there is a non-trivial quantity of people who are better. Candidates are compared and contrasted against the pool of other applicants; if there is someone else whose qualifications blow mine out of the water, or who is technically comparable but much cheaper, then who can blame Zapier for taking the obviously-superior option? I can't see the other applicants, so I can't know whether or not there was a slam-dunk applicant who made the merely "great" applicants pale in comparison.
Even if I'm highly qualified and there isn't anyone clearly superior in a comparable price range, people often can't quantify a firm rational basis for their specific preferences between candidates (and this becomes even more apparent once you're on the other side of the table).
This is when the infamous "culture fit" is broken out. I was once told that I had been passed over because I "wasn't assertive enough" after I spent most of an interview blasting the use of MySQL in a specific industrial application. There's no way that note wasn't intended as passive-aggressive retaliation. :P
Recently I was dismissed from a candidate pool because my answer to "Tell me about a time you resolved a problem with a colleague" hadn't been rehearsed to the satisfaction of the interviewer, who was one of those "I'm technical because I read WIRED and like being nerdy" types. That is just a different preference in technical depth as far as I'm concerned. Technically strong people are going to worry about technical interview content and focus on that rather than their answers to tired cliches.
Sometimes even things that seem trivial or silly can be a bigger impediment in context. For example, my first name is Jeff. I know of at least a couple of gigs that I've lost solely because there were already 3 or more Jeffs involved in the work on a daily basis and they didn't want to make the situation worse! "Jeff" is a somewhat common name but it's not super common -- sometimes I wonder how frequently this happens to Johns or Steves.
tl;dr it's just impossible to know whether a hiring decision is justified or not without having the insider dialogue and insight, and even then, don't expect stellar rationale. As bryanh said elsewhere upthread, hiring is a messy process.