It comes from the number of jobs unfilled. And yes we do have this problem.
Side note - I do NOT use whiteboard technique in my interviews. I ask the candidates questions about the stuff they mention in their resumes and go deep in the technical discussion based on their responses (or I told them to walk me through one of their recent projects be it a work project or their side project). Using that 'simple' technique, you can easily filter out so many pretenders (as I'd like to call them, who have splendid resumes but little to back things up when grilled the details about their resumes).
But asking about work projects specifically can be tricky, since you have to avoid any proprietary bits, to avoid any possible appearance of IP taint.
If they don't have open source projects to dig into, hypothetical plausible system problems (not leetcode exercises, nor puzzles) are an option.
You still have to watch out for people who know a lot of the right engineering things to say, and/or have done prep memorizing and drilling, rather than speaking from experience. (Example from one of the links in this thread, which had an example of a repetition flash card app, saying that UDP is connectionless. I don't know what good purpose would be served by having that info on a flash card for interview prep, other than to help someone who knows absolutely nothing about it to bluff their way through an interview that asks about it.)
From what I can tell, there's a sea of people who can barely do anything with some hand-holding, then each level of knowledge, competence and experience you want to go higher, you have to slice the pool by 100. By the time you get to level where people can competently drive non-trivial projects, there's a severe shortage.