The springs are plastic coated, and sit in a metal cup seat. As you drive the metal seat wears the plastic away at the contact point, then water the splashes into the cup wicks it's way up between the plastic coating and spring.
Do you live in a snow & salt area? We only get lightly salted here in KC but mine still broke at 67k. Folks in MN must eat them pretty regularly.
Another thing that probably causes them to die quickly is that there's often a opening in the plastic coating about 1 coil from the end. This results from the hanger they are put on when they are dipped in the coating. You can see it in the attached pic.
When you install new springs you can fix up the damaged area with some liquid electrical tape, and then orient that end up as well just as extra protection.
Also you can help your springs stay alive longer by cleaning up the lower spring cup and giving it a few coats of rubberized undercoating. That'll act a little bit like a rubber pad and reduce the damage to the spring.