The main issue is that exercise exacerbates hypoxia. You're climbing a mountain, your muscles need oxygen, but you're not getting enough.
But the temperature does make it worse. Your body is trying to generate heat, which increases oxygen demand. Your blood vessels are constricting, so circulation is less efficient, limiting oxygen distribution. All together, this creates all sorts of health risks.
Sitting in a comfy plane seat browsing the internet is not the same thing. Besides, even healthy adults just sitting can experience mild hypoxia effects above about 10k ft - fatigue, mild cognitive effects, headaches. But if you're just sitting, it's generally tolerable. Of course, you don't want pilots working in those conditions for any length of time.
But the temperature does make it worse. Your body is trying to generate heat, which increases oxygen demand. Your blood vessels are constricting, so circulation is less efficient, limiting oxygen distribution. All together, this creates all sorts of health risks.
Sitting in a comfy plane seat browsing the internet is not the same thing. Besides, even healthy adults just sitting can experience mild hypoxia effects above about 10k ft - fatigue, mild cognitive effects, headaches. But if you're just sitting, it's generally tolerable. Of course, you don't want pilots working in those conditions for any length of time.