This sort of thing is a natural consequence of high cost and high risk missions. You have a multi-billion dollar mission, and it's doing something unprecedented and using a brand new design and construction. When you build something like that what tends to happen is that you try to keep the mission scope reasonable and then you build the vehicle to have a very high (say >95%) chance of satisfying that minimum mission requirement. But what happens when, say, you create a Mars rover with a 95% chance of working for 90 days you'll likely end up engineering a vehicle which might have, say, a 50% chance of lasting for 5 years. If you set out to do that from the start it would be a non-starter, because a 50% mission success rate is too low for a multi-billion dollar mission.