Because the whole idea is based on overkill. For simplicity sake assume there is a 5% chance someone will find a way to make one of the legs obsolete overnight. Also assume that method doesn’t work for the others. Then there is only a 0.0125% chance that all three will be obsoleted together. There is however a nontrivial 14.3% that at least one of them will.
This is not obviously an oversimplified version but it boils down to not having to have a perfect system and three independent ones like this give you an outsized chance of been able to respond in a way that making the first move is a losing proposition even if you come up with some magic way to invalidate one of them.
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