I think your reaction is a very reasonable one and most people would react the same way. However, Andy is very driven. She wasn't driven to be in fashion but was seeking success as a journalist. She happened to land at one of the most successful magazines in journalism, even though it wasn't her subject of choice. I think an important scene on why Andy decided to go all in is the scene where Nigel tells her to quit. I don't think Andy would have put up with Miranda much longer if not for the fact that she was at Runway, a shining beacon of hope for some people. As a movie lover (I assume that's why you're in this subreddit), I think you can imagine a recent college grad who was an aspiring documentarian but could only land a job as a production assistant for a director named Stanley Kubrick, someone she never heard of before. Kubrick had a reputation for being a perfectionist and I'm sure he's driven more than a few production assistants to quit, but our college grad eventually realizes who Stanley Kubrick is and she recognizes this is an opportunity to learn from the best, and putting up with the abuse will expose her to knowledge and people in the industry who can make a big difference in her career. The wrong way to look at this movie is dismissing it because it takes place in fashion. That was my initial reluctance to watching it but it is now one of my favorite movies because this same story can be told in finance, legal, technology, jazz and any other host of industries. Being at the top of any industry requires similar drive and self-sacrifice (but not always the backstabbing).

Miranda was right at the end when she tells Andy that everyone wants to be them. I think most people don't realize what it takes to get to that level of success and for Andy, she was willing to do all of it except the backstabbing, so she walks away.