Yeah, but you don't need a thermometer for that.

Using zero as the freezing point of water makes sense for everyday use, and using the boiling point for 100 degrees does follow fairly naturally from that - but plenty of other potential reference points could have worked equally well.

That said, for all the American claims of "fahrenheit is more human", 100 °C is the temperature of a hot sauna so works much better as "the highest temperature you'll encounter in day to day life" than 100 °F.