Because Stephenie Meyer is a shitty 作者 who can't write to save her life. And she uses words that, make readers confused. She is using 随意 works like "beautiful, and sparkle, and such" to make the reader fall even 更多 in 爱情 with the vampires, such as "Edward Cullen."
In myth, 吸血鬼 are supposed to burn when sunlight reaches their skin. And also, they can be killed with wooden stake and they fear holy objects like the Holy Cross, garlic 花 etc. Stephanie Meyer moulded her version of 吸血鬼 so that they can go about easily instead of just coming out at night and also to make them seem indestructable. Meyer wanted to make her 吸血鬼 absolutely flawless, so she brought it up that her version of 吸血鬼 sparkle.