Power plants have been around even longer, and the fact that they don't have to be built to be mobile, lightweight, and cheap enough for an individual to buy means that all the effort went into improving efficiency and cost per watt. If you don't need to worry about the weigh of hauling around water, you can use your gas turbine's waste heat to run a steam turbine. And you can run at much higher temperatures than if you were running in a car, since you're now water cooled, and investing more into a single turbine.