Because keeping one's aircraft in tip-top shape seems to be of lesser importance when you're flying boxes instead of passengers. EPR/EGT limits exceeded? Oil leak? Hard landing? Heavy turbulence? Well boxes don't complain do they? The worst that usually happens is a tech stop or a precautionary landing, and only when one flies live animals or perishables does the time taken to delivery really matter.
Too bad sometimes things really do go very badly wrong.... The Bijlmerramp is an excellent example.
Not too long ago one of those white CAL/El-Al freighters we get frequently at SPL blew some tyres on takeoff from JFK and when it put the gear down on approach into SPL 06 it lost some bits of its wing-to-body fairing over a populated area. Just another day in the polder, I guess
