VW made several prototypes and a limited production model of a design concept for a car that would travel 100km on 1 litre of gasoline. An electric version could have a small, fast-charging battery and long range. Why isn't this a thing yet?

Because currently cost of goods would make it cost almost as much as a bigger car, making it look like terrible value.

I don't know what the op is suggesting, but this was a loss making technology demonstrator. $100,000 could buy you more car, electric or not.

This was conceptually more of a supercar, except concentrating on efficiency rather than speed.

Three main reasons I ask:

1. It may be easier to cost reduce a lightweight aero car body than a battery, where the costs come from raw materials.

2. Rapid charging is less critical to the usability of a vehicle with a small battery.

3. In many cases, battery replacement will cost more than the value of a 10-15 year old EV. A smaller battery is proportionately cheaper to replace.