All films are art but “art films” are a specific genre with specific general characteristics.
That is like saying “Every car is a vehicle but the term “vehicle car” will from now on be used to describe only sports car”. It is simply stupid usage of term for what it is trying to define.
You can say it’s stupid all you want, but it doesn’t change the fact that it is how it is used. Go ask people on the street if they think Jurassic Park is an art film and see how many say yes.
Just because something stupid has been established it does not mean one should just accept it. If you went on the street 500 years ago and asked people if the Earth is flat they would probably tell you “yes, it is”. It would not make it any less stupid.