Historically the American Cars and SUVs were different enough that it would be untenable to make them side by side. It's gotten better since the SUVs became more car-like, but they haven't invested in modularity like the Japanese automakers, sometimes the companies blame unions for the inflexibility, but I think that's a red herring. However, it may be the case that the unions make it expensive enough to build modular lines that it's just not worth pursuing.