Most of the new cars' (new means past 25 years or so) fuel gauge does not drop to zero when you switch off the ignition. On the contrary the temperature gauge needle drops down.
Local electricians call it "automatic" fuel gauge. It is not "always on". When the ignition is switched off, the fuel gauge circuit is also switched off but it retains the reading, and does not let the needle fall to zero. Evidence: If you disconnect the battery, the fuel needle still doesn't fall down, whereas it should have were the circuit energized 24x7.
One advantage of this is on bumpy roads, the fuel needle doesn't keep bouncing with the pitch and roll of vehicle. Also, you must have noted that needle of such cars with such arrangement takes a lot of time (2-3 minutes) to show full tank after refueling. This is because the needle moves gradually to compensate for bumpy roads, otherwise it would keep dancing and you wouldn't get a good idea of the actual fuel quantity.
AFAIK fuel gauge is a stepper/servo motor with no return spring. Installing a return spring would increase manufacturing cost while introducing a disadvantage (see above paragraph). Whereas temperature gauge is a simple potentiometer (could somebody confirm this?).
EDIT: In olden days maybe they sold vehicles with fuel gauges which returned to zero when parked because stealing of fuel was a big problem. And thieves would get an idea of fuel quantity just by peering through the window.