There are a few things here. Much like everyone else has said; attractiveness plays a part in it. It isn't something that's completely ignored, and there are plenty of studies out there showing how attractive people get treated and considered better. People will have a different reaction to an attractive person giving them a creepy pickup line, to a fat, sweaty old man giving them that same pickup line. Even if the reaction isn't positive for either, its often more extreme for the less attractive person, and if it would have been borderline for the non-attractive person, its got a fair chance to be acceptable for the attractive person. Humans, all people, tend to treat attractive people differently. Rather than hide from it, accept it, and realise its not a judgement about people, just something we do.
That said, another post kind of covered a key idea to this whole thing; the fact that it could potentially be creepy is why it can also be considered romantic. Taking that risk, of it being interpreted as creepy, for the chance of it being treated as endearing, is seen as romantic, putting yourself out there. I'd also speculate its a bit of a test; the main defining factor in whether someone will find it creepy or not is their tastes. It has nothing to do with reciprocity, or little to do with it. My partner loves me back as much as I love her, but were I to threaten to kill myself were she to not marry me, or to stand outside her house and blast a boombox of music... Yeah, she'd be pissed. Were I to insist on going everywhere with her, she'd consider me confining and dislike it. Others would find me devoted. It all depends on a person's perspective, and knowing where they draw that line between creepy and romantic, and being able to tread it properly so that you excite them but don't scare them, may be seen as a sign of intimacy, and of how close the two of you are. Step over it and you're not as close as you thought. Stay too safe and you're not really displaying any extra affection or understanding of them. Toe the line in a good way and you're demonstrating how well you know them, and being more exciting and interesting than just playing it safe all the time. Of course, even that's up to perspective.
The best answer to the question in general; who the two people are. What each of them values, what they individually find creepy and romantic, and how well they understand each other's feelings on that. That's all there is to it. There's a ton of factors that go into those sub topics like attractiveness and reciprocity of feelings and context and a million others, but each of them the specifics depend on the two individuals, and its the fact that they could be creepy or romantic that is often seen as attractive to watch and, occasionally, experience. Much like extreme sports and stunts in movies and such. A lot of people will watch the stunts and life threatening activities of Bond or a skydiver, and find it thrilling and cool. Not a damn chance they'd do it themselves though - well, maybe once every now and then for a rush to try it, if they are really interested in it, but its not something they'd like to experience too often. Its thrilling and exciting to put yourself at risk, but IRL its fucking terrifying and has dire consequences if things don't go right. That's what makes it so fun to watch; its something we might feel like experiencing occasionally in life, that we'd love to be able to do or find the idea of exciting, but won't because of all the risks associated with it [On the flipside with romantic gestures, things like blasting a boombox outside there house COULD be a sign that they're really devoted and love you and want to support you forever... Or it could be a sign they're insane, ultra-clingy and want to own you. Movies remove the risk of the negative, risky side of being impressed by such a gesture, leaving only the positives - again, also up to personal taste], and movies remove those risks, allowing us to enjoy them and remove the risks that make some of them become creepy - based on personal taste again.
Put simply, its a complex issue, as everything in this world is, and as with everything in the world, 90% of the complexity comes from the differences between people.