In part because manufacturers fail to identify niches underserved by phones. E.g. gopro-sized and gopro-robust, but unlike gopro/clones with stills as a core competency and not just as an afterthought (e.g. the entire stabilization approach of gopro-class devices does not apply at all to stills). I see a lot of people taking stills with a gopro in situations where they would consider their phone too bulky and/or too fragile. An RX100 will fit those situations no better than the phone.