The question might be asked "why would we assume the singer isn't singing about themself?" In modern pop music it's become it's become standard that the "character" an artist "plays" when they're performing or writing is themself; there's no suspension of disbelief like with a play where there's an entire context around knowing that what you're seeing is a portrayal. Same with a book or movie or other works of fiction, and with those forms of media it's clear when you're seeing or reading fiction or non-fiction.

In a sense, some artists write novels and some write memoirs.