It's a tool designed for professionals. Professionals operate with standards, so in a professional kitchen in America, you will never, ever need or want to read a temperature in Celsius. In the rest of the world, nobody will ever use Fahrenheit.

Having a button that changes it with a single press is actually an impediment to working quickly, and only introduces uncertainty and anxiety about the tool. Measuring temperature isn't like measuring weight - seconds matter. A moment of confusion could be enough to ruin a process, and a failure to notice the change in settings could create a dangerous situation. Nobody wants a button to see the wrong number.

Think of it like compile-time config. It's there, you can change it if you want, and everyone will have a preference, but if you need to change it often, you're not the target market.

In the rare consumer case in which you may be using foreign measurements you can change it once at the beginning of the recipe.