Some people genuinely like the idea of Zack and Tifa getting together, and in the abstract - devoid of canon and context, who knows: maybe they could be a good match.
They exist, but just tiny part of the western fandom i.e roleplayer.
I've definitely seen it thrown around that Zerith only exists to deny Clerith, though I don't know how good faith those claims are.
And it’s true the other way around: Cloti fans ship Zerith even though they either don’t actually have much investment in the pairing or they don’t actually like one of the characters in the pair. (Spoiler: It’s Aerith.)
And this is also relevant to the value system - "pairing the spares". No one likes to think that in their pursuit of their ideal "love" someone gets left behind or shattered. No one likes to think they have to be the bad guy in someone else's story. Hence this convenient - "I didn't do anything bad breaking up this relationship, because the leftover piece has found love. I didn't make him / her sad - they found their happiness."
I can guarantee you it's just a small part of the fandom. And yeah, as Zerith shippers, I have problem with this kind who just throw Zerith everywhere or speak for them only to deny Clerith for convenience. The large base genuinely loves Cloti & Zerith together regardless of which one you love/invested in the most as OTP. Idk if this was proper enough to be shared here, but I collect many of their parallel in the story to give you more understanding that shipping both equally works. Though, I don't give my thoughts of it like an essay, it's just pure quotes so free to discuss it.
https://eleamaya.tumblr.com%2Fpost%2F656955024427122688
some will invalidate Cloud and Tifa’s development by insisting he constantly wants to be with Aerith in the Promised Land instead of Tifa but I can’t tell how much of that is based on the creators’ intentions vs fan interpretations vs other writers’ interpretations in AUs.
That's why everyone is so hell bent on "canon" - because the creator said "Yes - that's what I think of love. You're right!"
Sadly, what Clerith thinks of love is the opposite of what Nojima/The Creator think it is.
Nojima values a deep and long-lasting relationship in hardship, a romantic relationship that needs to be maintained because once you confess your feelings and declare that "hey, we're official/canon now", it's not over yet. We know how Nojima depicts Cloti in Advent Children, they faced a relationship problem before finally together again and Nojima is sure of that. He also said in FF8 Ultimania, Squall/Rinoa is just 1/3 ahead of true love because.... they're still young and they just start, similarly he said in CC Ultimania Zack and Aerith are still young to call their romance is "true love" yet (I think he just refers to their relationship before 5 years separation because 89 letters she sends and how he want to return to her is deep).
Among these pairing, it's just Cloti who passes their 24-23 years of life, not a teenage romance anymore. He said, "I dont intend to write about love, marriage, family (laughs)" when describing Cloti. Squall x Rinoa, Tidus x Yuna, even Zack x Aerith don't reach that stage yet in the story since they're all 17 yo. But if Zack and Aerith could really survive at the end of FF7Remake, they would reach this stage too since their 5 years separation is the hardship they must conquer to be together again when they reunite. And if FFX-3 really came to reality as Toriyama said, I believe Tidus x Yuna would be together again since, in audio drama (FFX2.5), Nojima makes them break up. They will be 21 yo in FFX3. Well, only Squall x Rinoa that left to our imagination what kind of their relationship issue would be like since there's no sequel of FF8.
(I don't use other pairings because Nojima wrote only FF7, FF8, and FF10.... also FF Chaos lmao)
So, Clerith is really out of his mind of "canon pairing" or "true love" definition. But it doesn't mean he deny that the feelings already exist, Aerith's feelings for Cloud exist and he tells us in COLW. But Cloud's feeling? We never know and even if Cloud had, for Nojima too it seems, it doesn't matter since Cloud has Tifa and move on with his life. They're comrades, they care for each other, and they share "ai" (deep love that it doesn't matter whether it's romantic or not)... their bond is real, not fake, no matter how short it is (three weeks with Cloud not being himself till she dies and Aerith was searching Zack despite realizing to let go in the end). That's the bittersweet of Clerith and I would defend this from those who say Cloud and Aerith's bond was 'not real'.
But I know this would never be the ideal Clerith shippers want. They want their own version of Promised Land and would always argue that Cloud loves Aerith more (even only her since Tifa is just mere NPC lol) although we've already known this answer for so long.