Safety margins
It's worth noting that safety margins are pared far finer in the launch of unmanned rockets than in any other field of engineering I can think of. The rocket equations provide ample excuse. A small gain in rocket efficiency translates into a large gain of cargo weight delivered into orbit.
I don't know what happens in practice. In principle, too many launches without a failure should result in someone deciding how to shave more weight and safety margin off future launches!