I wanted to chime in, since I'm in the unique situation of not being a "Swiftie" but still having an above-average knowledge of Taylor Swift due to being married to a Swiftie.
For starters, her songs are very relatable for women. Especially in women around her age, she was routinely writing songs that spoke to the emotions during each periods of their lives. My wife, for example, was in middle/high school when Taylor was releasing her romantic country songs, and met me right around when Taylor released Lover. This is all because Taylor is extremely autobiographical with her lyrics and was writing about what she was experiencing at the time. She wrote lovesongs in Speak Now because she was in high school and early college when she produced the album. She wrote Lover because she had met a man who, at the time, she perceived to be a man she could spend the rest of her life with. Since Red, very few of her songs are about hypothetical situations. Almost all of them are about her real experiences as a person and as a woman, with the exception of folklore and Evermore, and that speaks to women in a very strong way. Her lyrics and reasons behinds songs are deep, much deeper than most give her credit for.
Additionally, she is extremely good at marketing. Many of her songs and albums have Easter eggs in them that only true fans will be able to find. She also drops a lot of cryptic hints, which her fans love to dissect and interpret to try and predict major releases or announcements. It's just good fun for them, and it increases the hype significantly. Also, her concerts are not just live music, they're a whole show. The Eras concert is 3 hours long, and she is singing and running the entire time. She rarely lip-syncs - I say rarely because I've heard claims that she does but I have never seen it - and gives it her all every single concert. Her band and many of her dancers and support staff have been with her for a decade or more now, and they have continued to routinely put on shows to the best of their abilities without fail.
Finally, she is, most Swifties believe, a genuinely good person. The worst thing I've ever heard of her doing is loaning her private jet out to her friends and families which caused her to break the news because her jet was causing a lot of emissions. Beyond that, she seems to be a grounded woman who genuinely loves her fans and the people around her.
If you take nothing else away from this post, this is the most important fact: She is relatable to women. She sings about her lived experiences, many of which are relatable to her fans.
