Because the pilots still have to be able to fly, which takes a lot of training, practice and certification. The skills called into play during a flying intervention are much more than the skills called into play by a driving intervention.
If a pilot walks, it's very expensive to replace him, because the pool of pilots are already trained and certified prior to a job app. An airline can't take anyone of the street and create a pilot. Or at least, they don't. Pilots are rare.
A trucking company can take anyone off the street and create a driver/monitor, in part because virtually everyone can already drive, even if they don't yet have a CDL. Drivers are not rare.
