Applicants are not necessarily qualified. There could be thousands of unqualified people applying for a job and at the same time a shortage of qualified applicants.
There are two reasons for that wide distribution:
1. Lack of preparation/expectation management. I have seen people apply for development jobs with no skills or experience thinking the job is there to train them. It is no surprise (except maybe to the applicant) that they are quickly disqualified. It is absurd how commonly this occurs.
I have also seen college educated computer science graduates get a job and completely struggle. There is some expectation among some of these guys that they deserve an awesome job, because they went to an awesome school, and everybody has always showered them with praise. The worst is when one of these guys get a job writing JavaScript only to discover it isn't anything like the Java they were taught at school. Instead of accepting reality it is simply easier to blame the language and do everything possible to make it behave and look like Java. epic fail.
2. Selective bias. How do you define competent or qualified? This is completely subjective and varies by organization. I have encountered several organizations where the developers cannot read code. They compensate with assertion testing, frameworks, external packages, layers of tooling and so forth. The unwillingness of a candidate to accept that stupidity is certainly an incompatibility and can be seen as a lack of competence. If the interviewing party likely struggles with literacy it is not something they are likely to test for, for example.
In my experience under-confident hiring teams always want super competent applicants until they see what super competence actually is. It sounds absolutely fantastic in theory until they see it in practice. Then the insecurity sets in. What is really tragic about that is not that incompatibilities happen, but that insecure teams often don't see it until far after they have wasted people's time or after they have hired the applicant only to wish they hadn't.
My recommendation for experienced developers is to always identify these incompatibilities as early as possible. Unless equity is attached this is so much more important than compensation.
---
In a perfect world people would learn to read and write code before applying for jobs, which implies literally reading and writing (not tooling or dicking around with layers of abstractions). We don't live in a perfect world as evident by many applicants and employers.