If they're paid higher than average, then how could a union fight to get them higher wages when they aren't doing it for the unionized competitors that have lower?
And also per another comment here, they require an unblemished driving record. So if the worst concern of unions came true, that they can make it hard to fire bad performers and require hiring "their guys", then drivers with blemished records can make it in and stick around. That means the good drivers currently employed may see their value drop comparatively and likewise their ability to demand better pay.
As it stands it seems like the current drivers then have the benefit of being considered best in industry and may prefer to gate-keep and not unionize as a result in order to protect themselves.
I wonder if other companies that want to fight unionization should do similar: hire top performers and pay them well :)