I like to think of it not as a question of girls generally but of this particular girl. For example, I am not an athletic person. My head seems to be a magnet for soccer balls and volleyballs. So in high school gym I would spend the whole time walking around reading a book. As long as i kept on moving, the gym teacher left me alone. In that context the question "why does she read?" would be fair.
It is, but you can tell this is 'she' because the verb पढ़ना has been conjugated to पढ़ती. 'Why does he study' would be वह क्यों पढ़ता है?
Thank you, Zeebo7. Your comment helped me identify the pattern and finish off this skill properly. I was not able to figure it out on my own.