Question

why we can't boil food on mountains

Answer 1

we can't because temperature is very low and it does not contain oxygen to put on fire.

Answer 2

$\small\mathcal\red{Why\:it\:is\:harder\:to\:boil\:water\:on\:top\:of\:a\: mountain?}$

At higher altitudes, air pressure is lower. When atmospheric pressure is lower, such as at a higher altitude, it takes less energy to bring water to the boiling point. Less energy means less heat, which means water will boil at a lower temperature at a higher altitude