Well, you could say litres/meter...but that would give you figures that are not terribly useful in daily life. That's why we use kilograms and kilometres instead of grams and metres...because the decimal system can handle powers of 10 nicely. 100km is a distance that makes the fuel consumption fall within a nice, easy-to-use range, and it's unfortunate that there's no handy term for 100000m like there is for 1000m.
km/l is useless as a measure of fuel consumption, because you're measuring against a constant; the fuel consumption will always be 1 litre. km/l is a measure of fuel efficiency, not fuel consumption. Fuel efficiency is useful as a theoretical measure for the efficiency of engines or whatever, but fuel consumption is more useful as a measure for how much fuel your car will use.