Are you able to provide any kind of data backing up that claim?
I am strongly in the "don't hate the player, hate the game" camp on this issue.
This is NOT a basic courtesy but a business process that requires a lot of resources to be at least somewhat meaningful. How long do you think it takes to write a rejection letter that provides useful, actionable data for the potential candidate? Multiply that by 10-100 for every position.
Oh, and I've seen much more nasty replies and general insults than thank yous in response to rejection letters.
I would be in support of rejection letters if the process would be mandated by the hiring platform / shared culture / etc.
[0] At Google's scale, some candidates fall through the cracks. But their recruiting process does mandate rejection notifications. It'd be great if a Google recruiter would back me up here :)
Maybe some companies do more personalized responses but I've never seen it and I would think it would often be difficult. As with many other things, say picking talks for conferences, you're usually not so much rejecting as you're picking some other person or thing. And, yes, there are a lot of reasons the typical HR department would have issues with brutal honesty even aside from the effort it would take.
[Edit] Holy shit, I'm old.
I see nothing in the post nor this thread that indicates that's what people are looking for. An obvious form letter would be just fine. The point is that a candidate wants to know that the process is over, instead of being left hanging.
I am strongly in the "that is a shitty excuse perpetuated by people to excuse their crappy behavior on a wide number of things" camp.