The longer the flight, the greater the accident rate
EXENG AF337 had been in the air for over four hours.
The longer the flight at altitudes like 35,000 feet, (at a cabin altitude of 8000 feet) the greater the likelihood that pilots will screw up - see J H Goode (Journal of Safety Research "Are pilots at risk of accidents due to fatigue?" Issue 34: Pages 309–313).
The pilots may feel fine - but they're not. It's not their fault. They haven't got enough oxygen in their bloodstreams. Their problem-solving capacity is degraded. Not a lot, but enough to make a difference in a complicated emergency.