It's not the case that there's a constant 1/5 probability of failure each year. Many failure modes are based on cumulative stress/degradation; so the probability of failure can go up over time.

Some failure modes go down over time; maybe there's some manufacturing defect, and those that have the defect fail early, while those that survive past the first year will have lower chances of failure early on.

But in this kind of environment, the cumulative stresses are much more likely than the early failures.