Because contrary to what most people seem to think, Hermione doesn’t look like this:
There is a huge disconnect between how people who only watched the movies envision Hermione, and those who grew up reading the books (not that I grew up reading the books; I didn’t. I’m just familiar with how things came out and it’s actually interesting to observe).
You could say that Emma Watson was a huge miscast. We understand why they did it, because movies are a visual medium and they wanted the main characters to look as attractive as possible to appeal to the audience.
But the truth is Hermione really just wasn’t the kind of hot cheerleader prom queen girl that the movies portray her as. Harry’s type is the athletic, active, sociable cheerleader type: Ginny Weasely.
A girl who is extremely pretty, with long straight hair, petite, and loves sports, particularly Quidditch, which was a big part of Harry’s campus life. Not an eccentric introverted squirrel girl like Hermione Granger. She is described as having big front teeth, big bushy hair, and a shrill and even annoying bossy voice.
You’ll note that his original love interest, Cho Chang, is also like Ginny. And nothing like Hermione. She is described as an extremely pretty girl with long shiny black hair. She was one of the queens of the campus.
Unlike Ginny and Cho, who were described as among the prettiest girls at Hogwarts, Hermione was not described as a pretty girl, and was not the campus queen type. I think the only time she was described as pretty was when she was all made up for the Yule Ball.
Hollywood is like that; the leading lady has to be cast to be prettier than the supporting actresses. That’s just how Hollywood works. In reality, Cho and Ginny were the hot beauties and much more attractive than Hermione.
But that wouldn’t sell the movie well, so in the movies we got this:
They have to cast the prettiest actress as Hermione, and the lesser actresses in the supporting roles. That’s just how movies and television work. You generally can’t have the supporting actresses look prettier and more attractive than the lead heroine.
But but men aren’t always attracted to the hot types! Can’t they fall in love with someone they’re always with like Harry and Hermione? Sure. But it didn’t happen here, and Harry friend-zoned Hermione really early on. Harry thinks of her as a sister, and not as a love interest. That’s just how it is.
A lot of this “Harry x Hermione” shipping is a result of the how movies have conditioned us to have the leading man get the leading girl. That’s why they cast the most handsome actor as the leading man, and the prettiest actress as the leading lady. But that’s not how it always works out in other mediums like books.
I’m sure there are some people who ended up shipping Harry and Hermione just from reading the books, but I can’t imagine there’d be that many. My girlfriend grew up reading the Harry Potter books and she never shipped Harry and Hermione. But if the Harry you grew up with was Daniel Radcliffe and the Hermione was Emma Watson, then it’s a lot easier to ship it that way. Especially since the movies emphasized Hermione a lot more compared to the books.
She looked like this: