e.g. volcanic ash is also an environmental trigger. But it is likely to certain to cause engine failure. No amount of ash is acceptable, but there is a density at which engine failure will happen quickly, and it's going to affect all engines at pretty much the same time. So in this extreme case, certain probability of engine failure in the triggering conditions means you have no redundancy with additional engines.

Whereas the problem sounds more variable and complex. Probably is low, therefore additional engines does provide redundancy just not as much redundancy as being in non-triggering conditions. Decent chance there's enough time to find another altitude to avoid additional failures.