Because I'm sure that's the only device he has with him ever, and never does he have an aide standing by that holds onto whatever phone the Government issued him.
You mouth breathing moron, do you realize that that the phone is a huge security risk without him ever making a single call on it? If it is an unsecured phone, it could very well be broadcasting POTUS's GPS coordinates constantly. This would give his exact position at all times away to anyone with hostile intent. Further, smartphones have microphones in them. If you root the phone, you could possibly listen in to conversations happening in the same room. What harm could occur if 'bad guys' (Trump's words) could start listening in on POTUS's confidential conversations? And who knows, Trump might just disregard the Secret Service instructions and use the phone sensitive calls. He seems to have established a track record for making poor judgements.
Yes, it is a very good use of elected officials time to make sure that a POTUS who wants to be a loose cannon is not allowed to jeopardize the safety of America.