Because it is the human scale for temperature. It turns out to not matter that much. but imagine a cold snowy winter, that's 0 and image a hot summer that's 100.
Really both C and F are silly, the minute temperature was found out to have a zero point, they should have been discarded, and we should have gone full in on Kelvin(or Rankine if that is your jam), or even better a new unit that better integrates 0, a scale from 0 to the boiling point of water? and give water boiling 1000? or would it be better to give water freezing 1000? Triple cells are a common way to calibrate temperature.
Really both C and F are silly, the minute temperature was found out to have a zero point, they should have been discarded, and we should have gone full in on Kelvin(or Rankine if that is your jam), or even better a new unit that better integrates 0, a scale from 0 to the boiling point of water? and give water boiling 1000? or would it be better to give water freezing 1000? Triple cells are a common way to calibrate temperature.