I think it's because fuel tank sensors frequently break, due to being buffeted by sloshing fuel, and sometimes getting corroded if there is a little salt water in the fuel, so car manufacturers have decided to use other methods other than a float and variable resistor...

I'm not sure what is actually used instead, but I'd guess they just measure the pressure on the inlet to the fuel pump. An absolute pressure sensor only costs 3 cents and the fuel pump is probably already on the CAN bus.