Those who can't, manage. Take a look at your station managers. Usually, they were the very worst couriers, You know, the ones that always whined and dumped stops, or skipped all of their next day E2 because they knew the swing would be doing the route? They were also the ones who would rat anyone out to management and talk crap behind your back....the manager's "pet" employee. They wanted to get off the road more than anything, because they already had a couple of accidents on-file and their customers hated them because they couldn't do the job right, much less establish a professional working relationship.
My personal favorite was someone we called "Fat Bastard", a loathsome creature if there ever was one. If there was a Cheeto on the floor, he could move like lightning, but the rest of the time he harassed couriers to be more productive and belittled anyone he could, mainly to soothe his own struggling ego and to compensate for his bloated appearance. The few times I ever saw his fat ass on the road, he cherry-picked stops that were easy and dumped the bulks and tough residentials.
There are always exceptions, but none of my current managers can do a full route by themselves, and they were all lousy couriers.