Because the vast majority of job interviews are with terrible candidates, even if the majority of candidates are excellent. This apparent paradox has a simple explanation: excellent candidates selectively apply to a few companies and get interviews/offers at almost all of them. On the other hand, terrible candidates are rejected at every step of the hiring process, and have to constantly reenter the interview pool.
Suppose 90% of candidates are excellent and 10% are terrible. If the excellent 90% only need to interview at one company, whereas the bad 10% need to interview at 20 companies, then only 0.9/(0.1*20+0.9)=31% of interviews will be with qualified candidates. To retierate: almost 70% of interviews will be with terrible candidates, even though 90% of people applying for jobs are excellent.
Because the cost of a bad hire is so consequential, the interview process is not designed to efficiently handle a minority of qualified candidates, but rather efficiently weed out a majority of horrible candidates. It is therefore a terrible process for the people actually qualified to pass it.