While not quite classics, they are based on classic fairy tales. Perfectly beautiful, desirable, all sweetness and light, Tragic Pasts, Twu Wuv, Cute Animal Friends, the whole package. The Sue-ishness might have had its roots in the original fairy tales, but Disney certainly popularized them, and many a little girl has wanted to be a Disney princess. Arguably, this is why some little girls grow up to write characters like that.
Interestingly enough, my friend on DeviantArt, who got mad when her characters were accused of being Sues - I posted on the Board about it - is a Disney princess fan, and when drawing her OCs in Disney style used the princesses as references. When she posted the pictures of them, she asked for critique, which I gave to her. (I'm not sure she's listening, but it hasn't gotten angry and I've offered to beta read any fic she writes, so hopefully I can stop anything before it gets onto the Internet.)

...to announce that I've discovered a new Sue color, known as Glite. It is an eyeball-melting, highly unlikely combination of gold and white, and I discovered it in this Warcraft fic.

It is now the official color of OFUA.

I never quite got around to finishing A Little Princess. Sara grated on me as a little Lily.

I'm sure Sir Galahad must be some form of Stu in some version of Arthurian legend, given how ~pure~ he's touted to be.

Thankfully, the writer is so good that the book is still entertaining despite Sara's Canon Sue status. It's almost like a literary experiment: If the worst thing in the world happened to the best person in the world, what would happen?

Back then, I think it was considered more okay than it is today to write about idealized characters, especially idealized child "role model" types. I don't think it was fooling anybody even then.

Charles Wallace... nah, I don't think he's a Stu. His intelligence is really the one remarkable thing about him--he's not given a bunch of superfluous positive traits for no good reason. Sara, for example, isn't just kindhearted (the only trait she actually needs to have for the story to work); she's also beautiful, intelligent, well-traveled, multilingual, and a great storyteller. That she eventually does crack, just a little, is the saving grace of the story.

Charles Wallace is just unusually smart. He's also small for his age, often bullied, sometimes overconfident, and somewhat autistic--more of a realistic character than Sara. Real-life prodigies and profoundly gifted kids really can be that smart.