There’s a difference between “realistic” and “not absolutely impossible”. In order to be realistic, a series should be not only non-impossible, but also reasonably plausible.
Harry Potter? Nuh uh.
Seriously. Magic all over the place, dragons, goblins, ghosts, giants, centaurs, mythical creatures that we know are made up being real after all. Some of which defy basic biology, like the centaurs that have two more limbs and a longer spine than every other vertebrate known to man.
Oh, and they’re half human and half horse, two species that evolved independently since a very remote common ancestor. How did that evolve? And does it have two sets of lungs, and does its food pass through two stomachs?
What’s more, this series uses magic as its variety of applied phlebotinum. Spells can do (damn near) anything, they follow no particular patterns, nobody ever explains how they work and nobody really cares how they work. I think it’s a safe bet that this means Mrs Rowling doesn’t really care about creating a consistent universe, if it gets in the way of having living paintings and staircases with a mind of their own.
And it’s a children’s series, so that’s cool.