Why do you need that requirement be validated by people and at a level not connection to the place of work?

Presumably 1. the "places of work" are not doing sufficient validation, and therefore 2. regulation is needed when the non-regulated path is failing.

Unless the places of work are vetted, setting up a company to offer a job, and collect fees for offering said "jobs", would seem to be a simple way of committing fraud in that case.

So either you vet the companies offering those jobs, or you vet the visa applicants.

Countries are not just places you work, first of all.

To prevent fraud. It's the same reason governments have driving tests and tax or grant audits. If the government deferred to applicants for everything, there'd be no point in the application process.

Sure, there's a libertarian argument against limiting visas, imposing taxes, and issuing grants, but if you are going to, it requires some amount of enforcement to prevent rampant fraud.

Because the government is responsible for border control and immigration?

The alternative is that the company must provide evidence, but I don't see how this is better.