I think a lot of people in this thread are too young to understand what is meant by attic. Go upstairs in a house built in 1920, that’s an attic. A tiny cubby in a cavity left by engineered trusses is not. You’ll also notice the modern house, even a cheap one, is far better insulated.

At least where I am, I would attribute it to code requirements. A livable attic requires much more work to meet code than a vented attic space. Not to mention the energy cost of having a livable attic can be higher because there is no venting and limited insulation. All of this is speculation of course. You definitely could do it. But, if you did you'd almost certainly have to use more expensive construction techniques to meet code insulation requirements.