the big difference is people voluntarily choose to own or keep this electronic device, and to fill it with their personal data etc.
its also not trivial to make a mobile device covertly transmit multimedia data regularly without running down the battery.
As well as this, it could be discovered even accidentally if it is left near a piece of audio equipment without RF shielding as a transmitting mobile phone makes a distinctive noise on the loudspeakers or headphones.
In any case in the same way you should inspect your bicycle or car to ensure it is working well and safe and (if you are concerned about enemies) that it has not been tampered with in any way, you should also do this with any vital communications or IT equipment to keep safe.
