Why do ovens use Fahrenheit instead of Celsius?

Baking ovens in the US still use Fahrenheit because that was the scale used by earlier ovens. Because Fahrenheit is still widely in use for cookbooks, recipes, and other food guides (such as turkey roasting), manufacturers are reluctant to change their products. Some newer and digital ovens are calibrated in both Celsius and Fahrenheit, especially those marketed outside the United States.

The general idea is that, eventually, both scales will be used, so that Fahrenheit can be phased out over a period of time. Fortunately the temperatures used in ovens (about 120°C to 240°C) are very different when expressed in Celsius.