The answers thus far are valid ones, but to REALLY understand why this is, stop by your local Cummins shop and ask to see one that is torn apart.

Once you look at the parts I think it will clear up a lot.

This is no mistake, it is common for these engines to go a million miles between overhauls. Also they take about 12 GALLONS of oil at change time. The filter is HUGE.

If you ever want a real thrill, try to get a chance to stand in the dyno room with one of these big Cummins when they load it down at full throttle. It is an experience that is totally impossible to describe. The only other thing I've ever done that compares is throwing a live hand grenade. Both of them SHAKE the ground and shake it SERIOUSLY.

Have a great day,