Not quite true in all cases, about a third of my van drivers are all ex bikers, but after on average 5 years of despatching on a bike through our wonderfull British summer, never mind winter they tend to migrate into a diesel van, which are cheaper to run than a bike anyway.
They still drive vans like despatch riders though!!!
just as an add on, the longest serving biker here is coming upto 20 years continuous service on a bike and currently uses a NC30, and before that had 3 Yam FZ750's, a bike renowned as having one of the worlds most uncomfortable seats. The highest mileage he did on one of the FZ's was somewhere in the region of 400,000 miles before it finally died.