To my feeble mind - I'm no radar expert, just some general E-M and antenna courses - thin sections (relative to radar wavelength) of many materials with low conductivity and relative permittivity may produce little reflection at normal incidence, but what about all that fuel in the tank. It's like a large 3D object and its permittivity is ~2, so it's going to reflect some. That's why the emphasis on radar absorbing materials for the skins.