Interviewing is not exactly top priority in a company. Sure, management will say that it is, but projects will in fact have priority, so it tends to be neglected and just grows into a series of decisions based on personal experience, internet stories and "best practices".

It really takes a lot of effort to be a competent interviewer. One needs engineering skills, but also soft skills, empathy, teaching skills.
Doing interviews doesn't count in one's performance evaluation, especially given that it's almost impossible to figure out whether an interview was performed in a good or just average way, so for the typical engineer it's a PITA with no upsides.
Candidates will be treated respectfully, but the process is what it is. There are no incentives to improve it, especially for companies that are in demand.

Just so you get an idea how hard it is to manage hiring, when we came up with our interview process we more or less had to do the following:

1. Selected a group of competent developers with interviewing experience. They were then responsible for the whole technical side of interviewing.

2. Convinced management to offer support for the initiative.

3. Evaluated multiple options on how to structure the interview and chose one (e.g. phone + several face to face).

4. Prepared guidelines on how to behave, overcome typical biases, evaluate candidates.

5. Prepared a pool of exercises.

6. Added verification steps to make sure that nothing went wrong in the phone interviews.

7. Ensured that there was always a mix of experienced vs. unexperienced interviewers in face to face interviews.

8. Set out some rules for offering feedback to interviewers after face to face interviews. I have to confess that this never really worked properly. Offering negative feedback is particularly difficult.

9. Checked all of the above with management and implemented.

Then we had to adjust based on practice:

* added guidelines on special situations such as timezone differences, interviewer mistakes, insufficient information gathered during the interview, replacements, interview preparation, offering feedback, offensive remarks, etc.

* scheduled weekly review rounds for all phone interviews and made sure that people took part and took things seriously

* tinkered a lot with the candidate funnel to ensure that interviewers aren't swamped by inadequate candidates but also that we don't miss competent candidates with unusual applications

* balanced between growth goals visible as management pressure to hire and ensuring that the process stayed true to its goal to demand certain technical standards from the candidates

* experimented with various ideas such as homework, different interview structures, laptop vs whiteboard vs paper, access vs no access to docs, etc

* made sure that office management personnel understood the above and was able to work with the process.

When you think about it, this was "just" a typical phone + face to face interview, not rocket science. And the description covered only the technical part, there was also HR, sponsoring conferences and job postings, managing invitations and organizing the appointments, expenses reimbursement, preparing reports for management, etc.

Such an interview method is not typically designed to find what's unique about each candidate and allow them to best express their individuality. It's a machine designed to answer to answer the question of whether someone is (probably) able to work productively in the organization.

The process starts decaying the moment it's not supervised. People come up with brilliant ideas about what could be improved (let's ask candidates to solve problems from programming competitions!), don't stick to the reviews, reuse leaked exercises, become too lenient or too harsh, don't prepare, etc.