The game does name Sapphire, but it doesn't name it Sapphire. Its real name is WEAPON, and it's the only individual monster whose name is supposed to be spelled in all-capitals like that. If you use Sense on the other Weapons when you fight them, you can see that they're called "-------- Weapon," not "-------- WEAPON." The official strategy guide incorrectly wrote all their names the all-capitals way, though, which I suppose is where the confusion started.

I still call'im Sapphire Weapon, tho'. 'Tis merely an aesthetic preference.
Originally posted by Phoenix 17
Diamond isn't that bright, he does actually just walk towards Midgar then stop.
He has to stop, or you might miss the fight.
Ultima just hangs around a lake till you attack it.
Again, game mechanics.
I don't think the planet would just make giant killing machines, without deciding what they were gonna do.
As has already been said, the Planet did decide what to have them do: tear up absolutely everything to make way for a fresh start.