Why don't job descriptions explain what the company does, and how?

The real reason is that most job descriptions are posted by recruiters, and they don't want you to find the company without going through them. They scrub them of any info that could be used to locate the careers page of the company.

For the same reasons that CVs dont say what people actually did in their previous jobs.

To be clear, your headline and description disagree. Do you care more about what the company does, or what the job will be?

Because things change. What you are hired to do may change the week you are hired (or even the week before).

So a company is going to tread the line between a job description that is so vague it won't appeal to anyone ("we build stuff ... using computers") and one so specific that it won't be accurate after the candidate accepts the offer.

To simplify your question:

Why does Bossbert write the Job Description - because he thinks he knows something.

Why does Catbert edit the Job Description - because if they do not then they have a hard time justifying their value.

Why does not Dilbert write the job description - because a job description that reads 20 pages is a bit much.

I recommend a website called Glassdoor. I used them to avoid a job, I was unemployed, and this company was interested in me. Reading on glassdoor, everyone thought it was the best place to work. Everyone except the IT people. They thought it was just atrocious, and the turn over rate was over 100% in IT. No thanks - I would prefer to remain unemployed verses entering that mess.

Alternatively you can go on linked in, see who works there, hopefully you know someone who knows them, get an introduction, and find the real dirt.

Most people writing job ads spend far too little mental effort to figure out how to write a job ad: they don't read 100s of existing job ads, they don't read a book or blog posts, they don't reflect on what they're trying to achieve at that stage of the process etc.

Instead, they copy and paste phrases from existing job descriptions (internal and external). The more specific the phrase, the more likely it's not relevant to the new job, so the less likely it gets copied.

So what does the job ad contain? The most abstract and meaningless phrases collected from a bunch of existing job descriptions.

Rinse and repeat, and you get to the situation we have today.

How they make the sausage doesn't really seem appropriate for job descriptions to me... but as far as filling roles goes I find that more often than not the people writing the job descriptions simply have a poor understanding of the role and the requirements needed to ensure success in that role.

I just wrote a job description.

https://codefor.cash/sally

Feedback welcome.

A large part of pre-interview hiring is handled by nontechnical individuals making 1/3 to 1/2 of what a software developer makes.

The job descriptions are vague so that the interviewers can insert arbitrary requirements during interviews in order to mask their bias.