She doesn't write the songs.
She and a team of songwriters write the songs.
Has there been a single hit that she wrote by herself?
Freddie Mercury wrote 10 of the 17 songs on Queen's greatest hits album, including: "Bohemian Rhapsody", "Somebody to Love", "We Are the Champions", "Killer Queen" and "Don't Stop Me Now". Two of these (Bo Rhap and Champions) are more than just songs, they are part of global popular culture...Swift and her team has never come close to one of those.
As is well documented her voice is weak. She also has no charisma or stage presence. Like with her songwriting she is aided by a team of collaborators who design the stage(s). In other words outside gimmicks separate to her.
Someone mentioned white woman...I do think this "wholesome" character is a factor for her success (somewhat akin to the gigantic leg up given to Elvis who sang songs from black artists who racist white america ignored, hence he wasn't named in the Time 100 people of the century). She is a middling 'artist' who (with her collaborators) co-writes cheesy pop songs for teen girls. She didn't register here (UK) for a decade like she did in the US. Now she is having her "cabbage patch kids craze" moment, as many others had (Spice Girls were a much bigger deal btw, with only Princess Diana rivalling them for the front page).
She's not terrible, she's one of the better products of the current God awful generation of music. That's also probably why she stands out more today. But comparing her to Freddie Mercury for example it just becomes silly to think of her as anything but a disposable cheesy pop act.