They also conveniently forget that Cloud's remorse was compounded by the deaths of Aerith *and* Zack, not just Aerith alone. And his depression and survivor's guilt didn't begin to exacerbate until Denzel as well as himself contracted Geostigma.
The more irrational fans also tend to disregard the fact that she's found her own happiness by being reunited with Zack and despite what they want to think, they still retain their love for one another. Unless they think that Aerith would dump Zack for Cloud or, even more hilariously, form a threesome lol.
Zerith is as heavily pushed in the compilation as Cloti tbh. Cloud places the Buster sword among her flowers at the end of the movie, Zack is never far from Aerith in Cloud's visions of them. Even he seems to know whats up.
I think a lot of the confusion comes from the "Maiden Who Travels The Planet." Novella, its canonical status is questionable, but it does feature Aerith being pretty dismissive of Zack and calling Cloud her "beloved." as it chronicals her time in the lifestream. But it also has her acknowleding Cloti soo....and also "Smiling like an affectionate mother." at them
But yea the reason Cloud leaves Tifa and the Kids prior to AC isn't because he wants to be with Aerith but because he feels like a miserable screw up. He's basically in the midst of a real life crisis, and goes off to die out of sight so he doesn't have to watch them suffer, nor have them watch himself suffer. He's a man trying to face his inevitable death alone essentially.