Why? It's not obvious that an employee works 2080 hours, for example, because they take vacation (or work extra), so charging a 2100th of salary doesn't cover vacation benefit. Also, the employer will pay part of health insurance, and that isn't included in the salary number, so why would it be covered by a calculation from the salary.

Once again, this relates to total comp not base salary alone. Total comp includes salary and benefits which can include vacation time.

So it's not 1/2100th of a base salary, it's the hourly rate x2100 to get the total comp.

Do you know what your preferred total comp is? Take the hourly rate being offered, multiply by x2100, if it meets that then it's a decent offer.

It's a useful rule of thumb but it's just a rule of thumb. It's not meant to be perfect but I find it easy to understand and calculate so it's served me well in my ~20 years of contracting.