The biggest reason for this is the float used to measure your gas level, which is a bulb on an arm like the one in your toilet tank. Bulbs are always going to be fatter than the arm, so you can go from full (float is completely submerged) to a little less full (float is floating on surface of gas but is touching the top of the tank) without the arm moving.
For the exact opposite reason, you can go from almost empty to empty without the arm moving.
So since your fuel needle is going to be in the same place for both full and slightly less full, the engineers chose to point it to full. The alternative is a fuel gauge that would never point to full and just confuses the driver. And since it's going to point to the same place for empty and almost empty, they chose to point it to empty.