Because regardless of the hotness of the market 90% of candidates are still completely unqualified for the position they are applying to. Companies would rather keep the role unfilled than hire someone who can't code "hello, world!" (and no, that's not an exaggeration).
That may be a challenge, but if a candidate has years of experience in the software industry then their resume might vouch for some skills. If a developer has delivered features to common tools that many appreciate then how is it possible that they cannot accomplish basic coding tasks? The main problem here is that teams that are hiring often don't even bother to read resumes and so they end up with a process that is expensive, broken, and actually insulting to the most qualified candidates.
That's the point of the interview, to validate the resume by having in-depth conversation about the projects in it.
Note this doesn't mean trying to test for the skills that they must've applied to do those jobs. That's hopeless to achieve in an interview for all the reasons endlessly documented in these threads. Instead, assume that if they did the jobs listed in the resume, obviously they must have the skills to have done them. So now the interview is about validating they did those jobs, not the technical detail itself.
Based on more than two decades of hiring, this works very well.
