I don't disagree but it's pretty rare, even when the submission is obviously through an applicant tracking system (taleo, brassring, etc).

I don't know why. They might be trying to avoid angry emails, they might be trying to hang on to some backup candidates in case the one the hired does not work out, or maybe the tracking systems really are as bad as they appear.

I went on an interview where they flew me across the country, put me up in a fancy hotel, and then...completely ghosted on me. I would guesstimate that they spent a few thousand dollars on me, including the flight, lodging, food, dinner with the team, and ~15ish hours of interviewer-time. I can't imagine that the cost of three minutes of the secretary (or manager)'s time would be a significant factor after that.

Regardless, it's annoying....

I think there is some fear is in responding negatively due to the vagaries of employment law, where you are giving people the opportunity to challenge you for not selecting you for the "wrong" reason. This is why most responses should be just,"we have not decided to bring you on at this time". I always like to keep a pipeline of potential candidates warm, contacting them at least once per month, so giving no feedback whatsoever seems like a bad strategy for anyone you might ever want to hire.

However, you have to just start to imagine all of the things that happen on the other side of the curtain. The hiring manager moves to another project or hadn't been assigned when they started interviewing, the recruiter changes or their contract ends, the project funding gets delayed, the technology choice changes, they didn't know what kind of person they wanted, etc. In larger companies, you need a lot of coordination so that you don't lose a good candidate due to their not being a fit on the project that got to them first.

I would never expect a response other than "resume received" from an online submission, but, once you speak with someone, a status update of some kind, even if it is a "no decision has been made" makes sense.

Still, I'm struggling to imagine how "Thank for you applying, but this position has been filled/cancelled" could expose a company to a lawsuit. You'll eventually assume you weren't hired after a while anyway...