"저는 공부를 합니다."
Because "I study" isn't necessarily present. It just means that you are a person that studies, while "I am studying" means you are literally in the process of studying.
저는 공부를 합니다 literally means "I do [my] studies"
To say I am studying(at the moment) you'd say 저는 공부하고 있어요.