it doesn’t actually ever say why they should. It completely ignores that it obviously takes longer to drive across into the other lane and then back than to pass the cyclists

Because it’s SAFER. Oh my god, have we really got so selfish that a human life is worth like a second.

As I’ve mentioned elsewhere, in the UK (which is where this image seems to be from), the “safe” passing distance for a car overtaking a bike is supposed to be 1.5m. Add that to the 0.5m minimum distance the cyclist is supposed to be from the kerb and the width of the cyclist themselves, and overtaking even a single cyclist should have the car almost entirely in the other lane anyway (UK lanes are typically narrower than their US counterparts).

Whether anyone actually follows those rules is another question, but that is how motorists are supposed to behave.

It is also written into our Highway Code that motorists should “give motorcyclists, cyclists and horse riders and horse drawn vehicles at least as much room as you would when overtaking a car”