Because a real pilot(tm) would not be affected by toxic fumes, so it can't possibly be that.
Jeepers. Toxic fumes are a real hazard when working around combustion engines and the reason that operators of planes, boats, and cars don't keel over every day is because of the concerted and unsung efforts of legions of engineers and technicians who work every day to ensure that breathable air is not contaminated with fatal air. Seals fail, pipes corrode, and sometimes there are bad designs that are more prone (even though it is a very slight risk) to leakage. With millions of flights, it is going to happen sometimes and the job of engineers and techs is to ensure that "sometimes" is a vanishingly small a number as humanly possible.