A slight factor differentiating security systems here is involved to the advantage of defenders: Attackers have to find a whole exploit chain, while defenders only need to fix one part of it.

The point is that, as the defender, you only have to find each hole once, while the attacker can spend an infinite amount of tokens trying to find more holes, that are increasingly harder to find and might, eventually, not exist at all. The defender can do that too, of course, but being in the defense, there is value in not being able to uncover new holes (your system keeps working, ostensibly) while as the attacker that's simply how you fail.