I do only one project at a time, too -- in theory, anyway. I have far too many large unquilted tops because quilting of very large projects is difficult to impossible, due to lack of space and equipment.
It's OCD, I think, that drives me to work like this, and I also don't have a dedicated sewing room. But I may have many more projects in my head while working on the current one. I can see that cutting and/or sewing several projects at once could be efficient, especially if you chain sewed strips or pieces. But that would be too assembly line for me, I think, and I wouldn't enjoy it. I want a beginning, middle, and end to everything I do -- well, with the exception of the actual quilting, which does lag behind, as I said.