It isn't simple being 'too close'. Allowing a mix of ranged and melee attacks when dual wielding would have required multiple sets of animations, and expanding the logic for handling attacks beyond what the game was originally designed for. I don't know if it wasn't done for design, technical or practical reasons, but it would have been a lot of extra work for configurations most people wouldn't try anyway (or at least the lack of it hasn't come up very much).
