Because it doesn’t really allow motorists to pass them more easily it allows them to pass them more dangerously.
When over taking a bike or a horse you should give it a full car width. When they’re on one side of the lane it encourages people to over take on single carriage roads when there isn’t enough space.
You can always spot people who can’t not be biased in these situations because they constantly turn it into us vs them when what is actually needed is understanding from the other person’s point of view.