Because depth of field depends on focal length as well as f number. Or to put it another way, it depends on the absolute diameter of the aperture, not the ratio

Because depth of field depends on focal length as well as f number. Or to put it another way, it depends on the absolute diameter of the aperture, not the ratio of the focal length to the aperture diameter (= the f number). (Also, of course, you can't stop down on a phone camera as it has a fixed aperture.)