Plus, who wants a full fuel cell in the car with them when it should/could be outside. I once saw a full fuel exterior tank unhinge and fly across the highway and explode one night at a big $$$ run and thank god it wasn't inside the trunk. It was on a 4 door 65' Nova that was quite fast.
Stock mountings can sometimes be stealthier and better than aftermarket placements and the plus side is, that you regain room where you need it in a street car. On a race car, it can mean the reverse but still be a safety factor.