Before you can understand how drug testing devices work, you have to understand how the human body processes THC.

In summary, THC enters the body in its ingested form, metabolized, and exits the body as 31 different metabolite concentrations. The most prevalent form of these metabolite concentrations is the 11-nor-9-carboxy-delta-9-tetrahydrocannabinol (aka THC-COOH) metabolite concentration.

Immunoassays, being non-selective testing devices that they are, recognize all 31 metabolite concentrations combined, instead of the metabolite concentration that matters the most (being the THC-COOH metabolite concentration). Because of this, assays are calibrated at a fixed cutoff threshold of 50 ng/ml. They are a "go/no-go" endeavor and have no ability to yield a quantitative value.

On the other hand, the GC/MS, being selective and more accurate, only recognize the metabolite concentration that matters the most (the THC-COOH metabolite concentration). In order to test negative on the GC/MS, only the THC-COOH metabolite concentration must register below 15 ng. Because the GC/MS only recognizes the metabolite concentration that matters the most (THC-COOH), the cutoff threshold has to be set numerically lower in order to make the two types of tests functionally equivalent.

Basically, what you are saying is, it's not worth the money for a company (even a fortune 500 pharmaceutical company like the one I am applying to) doing a pre-employment screening to spend the money out of the gate on a GC/MS
Even "Fortune 500" companies are bottom-line conscious (as with any line of business). To the corporate bean counters, the expense of using the GC/MS as the primary analysis on every Tom, Dick, & Harry on the company payroll is not justified. On a test involving multiple panels (drugs), it would run into the hundreds, if not thousands of dollars to run the GC/MS on all of the drugs on a given panel. The company, particularly one with a large payroll, would go belly-up and in short order.
The use of the Immunoassay as the primary analysis is much more pleasing to the bean counters because of its lower cost.

If the two tests are somewhat "functionally equivalent" what is the use of GC/MS?
The GC/MS is used as the confirmatory analysis because of its accuracy, and most importantly, its ability to provide quantitative values. A positive result should never be substantiated nor accepted devoid of a quantitative value.