They could design the fuel tank to be symmetrical about the axis parallel to the car’s axels. This would let it be flipped during installation at the factory to have the refueling port facing either side. Then the only difference would be the body panel and little door that covers the gas cap.
Many (mostly European and North American) manufacturers can’t even be bothered flipping the indicator and light controls around, there’s no way they’d flip the whole fuel tank.
They could but there are downstream packaging compromises that would cause. It is easier to design the vehicle without imposing that design constraint on yourself
