When you engage in combat, the Chocobo will close in an attack from the direction he's coming from and parks it there until the mob is dead. Since they're usually behind you when you engage, of course they're going to go right up next to you if you stand still.

You want him out of frontal cones? YOU move and swing the mob around. That's kind of your job as a tank. It's something done in literally every dungeon trash and boss in the game where it's possible.

It might make sense to you to have him not stand in aoes, but you're a person who is intelligent enough to know to do that. A chocobo isn't.

Their eyes are on opposite sides of their heads. They LITERALLY cannot see much in front of them!