It’s very similar to the discussion of self hosted vs cloud providers. Business people are willing to pay more for a predictable cost, rather than save a significant chunk of change but run the risk of having an unpredicted event that ends up with a large extra cost (such as replacing a whole rack tower
And in some large film unions, you need to pass a test just to get into the department, let alone run a generator.
They would be extraordinarily cheaper if they were working nonstop, but the asks are so intermittent they have to charge enough to maintain availability of the service. This becomes especially true and high cost-of-living areas.
You aren't paying for the machine, you're paying for a guarantee that there will be a working machine when and where you need it.