Whether you perceive people to be rude or pretentious really depends on your own perceptions and the people you're encountering. Los Angeles County is the most populous county in the United States, and the socioeconomics, culture, and general attitudes can wildly vary depending on where you are and who you're encountering. What is considered to be boorish or unacceptable behavior by some is normal to others. Also, some people who are actually very courteous and kind can act poorly at times of stress. If Emily Post had been stuck in hellacious traffic on the 405 on the way to something important she might have forgotten her manners for a while, too. A lot of the complaints about LA's rudest behavior is based on how we act as drivers, which can sometimes differ from how we act in face-to-face interactions. We have crowded highways and streets, a ridiculous lack of signals for turning left, and other challenges and frustrations of a huge urban city. At the end of a long day a lot of people are just ready to be home, and they might lose their patience with the newbie who doesn't know that you have to advance then wait to turn left. Aggressive driving, honking, road rage, and other forms of uncivil behavior aren't just on the roadways of LA. When I was in Beijing where the traffic makes LA's look like that of a sleepy little town in comparison, the behavior was outrageous, but I wouldn't categorize the people of Beijing as louts. I mean, our kindly driver snapped and drove *on a sidewalk* when he got frustrated with traffic. Another driver actually stopped our van and got out and fought with this motorcyclist who cut us off. The worst was brought out of them.
In some parts of LA pretentiousness, entitlement, and gold-digging are rare, and in others they're far more common. For example, from what Obviousman has written about his life in his answers I've read, he lives in a pleasant suburb of LA where most people are living nice, honest, normal lives far removed from the sycophants, fame-seekers, and snobs that tend to live elsewhere in LA. I wouldn't think too many people who are working in the entertainment industry and caring about titles within in would live down there. Last spring my dad and stepmom unfortunately moved to one of the most outrageously ostentatious communities in LA, and so my many of neighbors probably have different mentalities than his neighbors. To give you an idea as to how classist these people in this community are, they engaged in a drawn-out lawsuit to make the gardeners, housekeepers, nannies, and other "hired help" drive seven miles out of the way to use a different gate entrance than the one for residents. The community is actually not located within the city limits of Beverly Hills but has a BH zip code purely for the sake of pretension. You see moms with $500 diaper bags from PoshTots and preschool children in expensive clothing they'll grow out in a few months. You cannot wash your car in your driveway because that's viewed as bourgeois. Seriously, it's a rule. That sort of absurdity is not found in most of LA, and most people who live in LA would be revolted by it.
The image of "LA" that is broadcast to the rest of the world really isn't true to life. Most associate LA with Hollywood, and Hollywood with the portrayals of it in movies and TV shows. They think of it as being a land filled with people who act in a Kardashian-esque manner. So, maybe that's why people mistakingly view LA as being pretentious. In reality, only a tiny percentage of people have lives that bear any resemblance at all to what's shown on reality shows about life here.
Variations of this question have been asked here a few other times. I'm posting a link below where you can read other responses.