Because nobody puts it in the first part. Mass effect didn't, Harry Potter didn't. It's is ugly to call something "part x".
Mass Effect isn't a remake of an original game.
If there was an original Mass Effect 20 years ago that covered the entire trilogy and was just called Mass Effect, and then you get a remake 20 years later, called Mass Effect Remake, you wouldn't assume that the game stopped after Saren's attack on the Citadel.
Also, it's an odd decision from Square. They get bad press, cuz it's a scummy thing to do, but if their goal was to "trick" anyone, well, I doubt that there will be a significant number of people buying this game thinking it's the whole thing.
"Final Fantasy VII Remake" isn't the type of game that the average consumer just picks up while passing by a gamestore and says: "im gonna blind buy this".
If they're a fan of the original, I assume they'll make some research.