You need 2 metres clearance to overtake a single bike which is probably 2 metres from the kerb, so total 4 metres from kerb.
If they're two abreast, you need 2 metres clearance from the outside bike, which itself should be 2 metres from the inside one, which should be 2 metres from the kerb, so 6 metres in total.
There are plenty of places where you could overtake with a distance of 4 metres from the kerb, i.e. one bike width plus safety allowance, but far fewer places where there's 6 metres from the kerb.
Plenty of places where you could safely overtake a couple of bikes in single file, but not enough space to overtake a couple riding abreast.
"Sometimes", it's safer for bikes to ride two abreast, but sometimes not. I really don't like the way some cyclists just default to two abreast regardless of road conditions, road width, other traffic, central white lines, etc., "just because they can" - they're typically the arrogant lycra louts.