Why are pretty girls who are shy considered stuck up?

I think it's mainly because people associate certain things about a shy individual as being intentional, instead of realizing it's just something shy people do unconsciously. For example the hot and cold game. Shy people don't play this game, they may behave in a certain way which gives the impression they are playing hot and cold. They are not even aware of it.

People don't understand how shy people work, their behavior and their complexity. A shy girl (or guy) will much less communicate with people outside their clique, and they feel much more comfortable being around their friends. This will make them behave different with people they know well, and people they don't know well or at all. This will make them less likely to acknowledge strangers and avoid behavior that will invite strangers to approach them. It will look like they think their belongings, their friends are the best around because around them they feel most comfortable. The unknown is just something they don't like and that's why they will long for what's familiar.

This is a small example, and some people will consider that individual to be stuck up. It's because they have a lack of understanding of shy people and assume things way too fast, and probably also have some prejudices.

To be honest, there have been some people I felt insulted by and I thought they were really mean, but then I figured out they were just shy.

BUT: I recently met a girl who was in my class and colleague of mine informed me about the 'fact' that she was very shy and insecure and was crying about something she was insecure of and fear for failing a test. Now I have worked with this girl and observed her for a while and noticed she was just acting to be shy and insecure, probably hoping I would develop empathy towards her and make her pass the course. Luckily I also spotted her outside the classroom setting, when she was comfortable and I could see with my own two eyes she was not shy nor insecure, none of that: she was faking it. She was trying to manipulate me + she was actually stuck up.

This example shows that people who appear to be shy can really be stuck up, but I don't think my example is that common though. The answer to your question is mainly that people do not understand shy people, and just make assumptions about them without critical thinking.

I think it's shy people in general, not just the pretty ones. Part of the reason why, depends on how you handle your shyness. If you don't talk to people when they talk to you, that comes across as stuck up. If you avoid eye contact as well, you look extra stuck up. It makes people think you're blatantly, and intentionally ignoring them, even if that's not the case. If you avoid people, it makes you seem antisocial. If you blend into the background, like some shy girls who aren't as pretty, they tend to think those types are introverted.

These are just a couple of examples. But I think that if you can push past shyness just enough so that you are actually talking to people, it would help a lot. Even if you're mumbling or stammering, or if you're not quite getting it out there, you're at least talking, and you being shy or insecure comes across more clearly. Y'see, if you can talk to people, even a little bit, people might notice you being shy more. They could be sympathetic. They might even find it cute. Especially if there's a blush. But the other types of shyness listed above, doesn't help anyone.

I know this sounds like a lot to ask, but you really should put in an effort. Unless you have social anxiety disorder, chances are you can push past it. Even then, if you don't try, your problem will only worsen over time. So at least try. What's they worst that can happen? They laugh? They're probably not laughing at you. They probably are just laughing at the situation. You think they'll get mean? Then they're not good people. Not patient, and not worth your time.

Anyway, I kinda soapboxed a little here. Sorry about that. But yeah, those are some theories, and a few suggestions on how you could maybe change the way you are seen, by a little bit. At bare minimum maybe it can make you rethink how people might perceive things.

Good luck.