And that’s an extreme case. How many less extreme ones happen?
In the US alone, there’s research estimating the number of air fume events is around 2000 per year. The number of reported fume events is less than 10.
Mental degradation is insidious. “Just wear an oxygen mas-” what if you forgot about the oxygen mask. You forgot to drop the gear.
Extensive training (to the point of automation) and human resilience is perhaps the main reason fumes do not seem to be causing many incidents, but resilience can be individual and training cannot drill into pilots the correct intuitive response to all possible scenarios. In addition, it’s unknown in what number of incidents where the cause is pilot’s mistake that mistake was in turn caused by partial mental incapacitation (that perhaps not even the pilot was aware of).