I think you are mixing up different RPG conventions.
In turn-based RPG, you have to give commands for each and every party member, so you control all chars becos it's turn-based there is time for you to do so. Same for the semi-turn-based, ATB type.
In action RPG, combat is in real time. Most games only let you control one char, the rest of the party are AI, you cannot swap on the fly to control another member.
Unlike previous FF games, FF 15 is the latter. If you know of other action RPGs that let you swap char on the fly during combat, do list them (off the top of my head I can only think of Dragon Quest Heroes and Dragon Age Inquisition).