Body cameras aren't hidden and are worn by public officials while on duty, doorbell cameras are no more invasive than an CCTV camera a home owner might have installed on their premise.

I think the difference is that these cameras are relatively concealed, and can be used to record every interaction, even in pretty intimate/private settings. Yes you could do this with a cell phone but it would be pretty obvious your recording if you're trying to get more than just the audio of an interaction.