They can't and don't fly runway to runway by themselves because even the smallest problem/perturbation/deviation from the norm knocks the plane out of the higher levels of automation and requires a human to intervene. The automation is great at making small adjustments to keep a plane flying stable, but give it a complicated, unknown airframe and it would fail miserably
Fog yes, but one of the three redundant guidance units malfunctioning? No. In fact, pretty much any hardware problem or malfunction will prohibit CatIIIb landing, which is common in current aviation. Craft will regularly fly with "minor" hardware or sensors in an inop status.
Basically to let the plane land itself, first the stars have to align
