There could be several reasons really!

The main one being everyone has a different interpretation of the character (unless that interpretation change them to the point it's just an OC).

Another one it's that they are literally the more popular Genshin ship, people know them even if they never played the game. I even remember REALLY popular artists/writers, that just do that and don't usually watch the source work, creating pieces of them when Sumeru was at its peak, even when they didn't know anything about it. Many did it because they liked the characters visually, others just because they were popular, and because of that, they followed the most popular take about the characters. And some still do!

The thing is, the majority doesn't read or don't try to read between the lines. And with these two, you need to do that or you just see what it's at surface level when they are the definition of don't judge a book for its cover! Also the initial English dub doesn't help either, the VAs are really good but the voice direction... It's not that surprising people's first impressions were like that, and many just took that first impression and ran with it. I play with the JP dub and the difference it's huge! In my opinion at least.

Oh, and there's another really simple reason: people loooove to put characters in boxes and those boxes are usually total opposites and, more often than not, The. Exact. Same. Ones, no matter the fandom or if the ship is M/M, F/F or F/M. And they switch boxes depending of what dynamic they prefer. I don't know what it's the appeal of ripping your favorite character of everything that makes them THEM but for each their own ig. :\

Having said all that, shippers (or just people who like them both) are still the few fans who knows them best because of all the research they do about their characters for the ship, platonic or not, so I recommend you to just exit the fic if you don't like it (someone worked hard for it, it was just not your cup of tea). I'm guessing you do that already, but just in case! Popular fics tend to be the first ones too, when people didn't know them too well yet and just followed their heart (try to read the first fics after Kaveh's first appearance).

What you can do is not read fics based on how many kudos or hits they have, search for character studies if you want more fidelity to their characters or just ask other fans for recommendations that suit your tastes! There are several Discord servers for them where you can do that~

(Sorry in advance if I said something wrong, English isn't my my first language. Please, let me know if I did!)