Nobody ever said you did, but billions of people don’t live at sea level and because of that water doesn’t boil or freeze at 0 and 100 for them.

The problem with Celsius and it’s relation to the freezing and boiling point of water is that water is a rare chemical that is less dense as a solid so it will freeze AND boil at different temperatures depending on pressure.

Freezing temperature isn’t 0 at 1500 meters and it doesn’t boil at 100 either.

Edit to add examples:

On Mount Everest water boils at 72 degrees

In Denver, Colorado water boils at 94 degrees