Least and most intersted i've been in getting into Fate lol
Didn't think this could get any weirder but I guess here we are. I imagine the mobile game shenanigans are a major part of this?
The Mobile Fate game, Grand Order, is definitely one of the biggest offenders. When Fate first came out, Saber Alter was merely a corrupted Saber during the course of the Visual Novel and Saber Lily was merely what she looked like before she got to where she is now.
Than Fate Extra came out with Nero, which started a similarity joke between all of them while the one below her was merely a costume for her in CCC. Also, some of the things you saw were merely artist designs or costumes for the side stuff, rather than actual Servants. And Mordred already looked like Arthuria as she was cloned from her.
But it was Grand Order that decided to make all the things you see as their own Servants with their own personalities and due to Player bias, all of them can have the hots for your Avatar character.
While some are just the characters after events or with costumes or simply spinoff stuff, Nero and Okita canonly look like her for some reason, Mordred resembles her because she was cloned from Arthuria, and Jeanne just resembles her rather than look directly like her. And Lancer Arthuria is her if she was summoned as a Lancer rather than a Saber. But like I said, despite being costumes or whatnot, Grand Order decided to make them all their own Servant, which off course leads to having a harem of look-a-likes.
Oh hey it got weirder
IT GOT WEIRDER
Oh believe me. IT GETS WEIRDER!
Especially when Cthulu and HP.Lovecraft Older Ones get added in.
You know what else? The franchise started out as hentai games!
The only reason they did was because at the time, all Visual Novels were doing
scenes and so they were actually forcibly added. When Fate got popular and the Visual Novel was remade for PS2, Vita and PC, they took out the scenes.