>>716461393 (OP)Cuz the story was written as they went, and Zack was written like 80% through the game by Nojima, just months away from it being Gold. The way she's initially written is that she and Cloud come from the same town, suffer a bad fate at the hands of Shinra and Sephiroth so they have a common goal. She's a bar owner, and eco-terrorist and has a lot of agency.
As written, the promise at the well would be brought up once, and just is relevant to the first part. The genius part is that by creating Zack, they have it so that the promise at the well is THE most important event in the game and the entire plot comes full circle.
The downside is that Tifa goes from someone with agency to someone who ONLY cares about Cloud, and has subsequently built her entire life around Cloud. Bombing mission against Shinra? Only done to keep Cloud around and see what's wrong with him.
She becomes Cloud's mommy, fulfilling her Misato role in the story.