Well, fundamentally, external BB chainsets are all designed to be used with existing BBs which all (except a few like Hope who don't actually make chainsets) use plastic spacers to allow for misalignment and also to allow the chainset to fit easily (eg it just slides through the BB – compare to say replacing bearings on a hope hub where you have to hammer the axle onto the bearings or visa versa).
If external BBs were designed to be used on properly faced and aligned BB threads (note, not the same as 'facing' which only ensures that each side of the BB's face is in line with the threads on that side rather than ensuring that the BB threads are perfectly aligned on both sides) and also a much tigher/interference fit (requiring fitting the chainset with a hammer basically) so that there was no movement between the BB bearing and the axle then metal on metal would be fine and preferable really.
All the current designs compromise to ensure easier fitting and less requirement for tight tolerances and proper facing/thread alignment which incidentally is fine by me as it does seem to work ok IME in the real world.