My parents have a new IKEA fridge, and it also has buttons in degrees Celsius. But as I mentioned below, until quite recently, the knob mechanically influenced

My parents have a new IKEA fridge, and it also has buttons in degrees Celsius. But as I mentioned below, until quite recently, the knob mechanically influenced a bimetallic strip which shut off the power when it became too cold. You cannot expect precise temperature control from this setup, and any calibration would likely be lost after some months of wear & tear. So printing exact temperature measurements on the knob would've given a false sense of precision. I also suspect that at least in some jurisdictions, some governmental body would require exact and lasting calibration if exact temperatures were printed on the knob. At least where I come from, thermometers used for food must be calibrated and approved by a governmental authority.