Why doesn't Harry take a bath, brush his teeth, visit the optometrist, go to the hairdresser, or eat healthy food?
When writing a book, the author must choose to include events that are crucial to the plot and what he or she wants to reader to know. By including more menial daily activities, you tend to loose the interest of the reader. I don't know about you, but I don't think I would be interested in reading about Harry brushing his teeth.
Although, be careful to remember that Rowling does mention some of these details throughout the books. Harry uses the prefect's bathroom to solve the mystery of the egg in the Goblet of Fire. In the Sorcerer's Stone, it is mentioned about his unruly hair and how his aunt tried to give him a hair cut. The next morning it had grown back.
I am sure that Harry does these daily menial tasks, but J. K. Rowling probably didn't see fit to add them into the plot. What for? As for the optometrist and diet, his aunt and uncle did not have Harry's best interests at heart. Look at Dudley and you'll see how much nutrition meant to the Dursleys anyhow.
Rowling doesn't explicitly state all these menial tasks. But they are mentioned in minor details throughout the books so if you are desperate for normalcy in Harry Potter, hunt for those details! That said, we do get explicit information that Harry cooks, cleans, and gardens for the Dursley's and that is somewhat of a menial task.