It doesn't matter where the fuel source was located originally: a debris impact can significantly relocate a fuel line. Some engine bays aren't very big, particularly in small cars, so the move doesn't need to be significant. Wind at highway speeds also tends to aerosol dripping fluids extremely well which leads to an easily ignitable air:fuel ratio and many fuel-coated surfaces. Add multiple high-heat sources to the mix (exhaust, catalytic converter, etc) and things happen in even the best designed cars.
Simply put, debris impact causes rather unpredictable results. You do what you can to mitigate the risk but you cannot eliminate risk from an unknown impact. Tesla is doing a pretty damn good job at minimizing damage done and protecting human life.