First of all, fuel is measured in kilograms in F1, because it’s density changes with temperature, so litres would be innacurate. Secondly, lbs is an imperial unit, SI (international system) is used in engineering applications everywhere but the United States (and some other small African country).
Thirdly, and this is the most important. Weight is the gravitational force of a mass. A 10 kg mass of fuel is the same anywhere in the universe, but it’s weight changes depending on the gravitational force exerted on it. That same 10kg mass of fuel on the moon will weigh approximately 1/6th of what it does on earth (1.666kg)
The G forces in a Formula One car will change the weight of the fuel in the car as it’s going over crests, dips, bumps. The fuel is moving in the fuel tank all the time, which means that getting an accurate weight reading would be challenging.
Also, fuel flow needs to be monitored anyway by the ECU to determine the correct air/fuel mixture. That bit of data can be used to determine fuel consumption as well. A set of scales would be redundant, adding unnecessary mass to the car.
In aircraft, they also use fuel flow to determine consumption, not scales. Again, it’s impractical to use scales to weigh the fuel (as it’s stored in the wings), and would add extra weight (which isn’t a good idea for an aircraft). They use fuel mass, because that’s the critical measurement for aircraft for determining take off weight, and everything else (passengers, cargo etc) is expressed as a weight, so also expressing fuel as a weight saves converting units.