According to another commenter, 2x4 is the dimension of the rough-cut lumber. Traditionally, your carpenter would buy rough-cut, then mill it down himself and allow it to dry. Eventually we figured out that shipping already-finished lumber made more sense. So they sold “finished 2x4s”, ie, 2x4s as they are after the finishing process. And because the actual dimensions of the sold lumber matter more than the name of the lumber, this just remains as an artifact. It would be very annoying if suddenly every 2x4 sold in stores was actually 2"x4", since now you have to shave off a half inch of every board if you are replacing a stud. And calling a piece of lumber a “inch and a half by three and a half” would be onerous. So the dimensions stay the same, and the name stays the same. At the end of the day, it’s a thing that might trip you up once when you first get started in carpentry, and then you learn and it literally doesnt matter at all from a practical perspective.