Because it’s a really bad and impractical idea, honestly. Most gas generators don’t produce “clean” enough electricity to charge a car, unless it’s an expensive inverter. Forgive the oversimplification, I forget the proper electrical engineering terminology, but a regular gas generator will fry sensitive electronics. EV chargers will refuse to pull current from such a power source.

Gas generators run for hours on half a gallon of gas because you’re usually running a fridge, a furnace blower, and some lights off it. Average load of 100W or so? The Model X has a 100kWh battery.

There’s no free lunch here. A Tesla is only going to get 20 mpg or so when you account for the inherent inefficiencies to this process. You’re also carrying a lot of weight and volume for something that isn’t going to happen unless you’re an idiot. I don’t carry around a jerry can of gas in the winter. I just don’t go below half a tank. Same principle applies here.

The reverse process, where you use your car battery in a power outage as energy storage, is much more sensible.

If you insist on this, they have PHEVs that basically have a built in gas generator. And they’re usually cheaper, as they can get away with using a much smaller battery. Most driving is done for shorter trips, after all.

TLDR: the math doesn’t check out.