Or sidemount.
Buddy separation occurs. There was an incident written up and shared on FB where some highly trained divers (GUE T1+ if I'm not mistaken) got separated and had some other issues (not gas loss). If people that well trained can get separated, what hopes do the rest of us have? Hence single tank limits the conditions in which one can dive. And yes, I'm not as good of a diver than someone with GUE T1 certification. That should be obvious.
Accidents and buddy loss do happen, but your story does prove my point. Suppose one plans their dive to include a minimum gas calculation, SCR, and waypoints of the dive to check those things. In that case, one significantly mitigates the dive's risk and does not need to lug around a dead raccoon (pony bottle) with them unless required for decompression. Moreover, if additional gas is needed to conduct the dive, one should learn how to dive in a doubles configuration adequately.