Because the amount of current needed determines the SIZE generator you need to turn. Once the SIZE of the generator is figured out you determine the "cost" to turn the windings. generators run at a specific RPM , if you need more power you don't turn it faster you bring more generators on line. The cost to run THAT one generator does not change as the electrical load changes.
This isn't that hard to understand...
Im absolutely amazed at peoples lack of understanding on this...
don't they teach this stuff in high school any more?
I say again, you don't know what you're talking about. Two things go into the power it takes to turn a generator, first, the friction losses, and second, the work (torque x RPM) that corresponds to the energy delivered by the generator. Listen to a diesel-powered generator, running an arc welder. The engine strains more when the arc is on, and certainly uses more fuel.
You are not just wrong, but deluded to think that once you overcome friction for a given sized generator, the mechanical load is otherwise independent of the electrical power delivered. It's akin to saying that a car engine consumes no more fuel idling at a given RPM, than it does towing a heavy load in high gear at freeway speed, with the same RPM - after all, the friction in the engine is just the same in both cases.