Because the image assumes that a driver can only ever safely overtake if they’re completely in the other lane, which simply isn’t true.
It also assumes that there will be an opportunity where the other lane is completely free for them to move into it.
Overtaking eight people in a line is going to have a large time saving if they’re cycling in twos, but when you scale that down to just two then the difference is negligible and the space saving is more important.