Why don't some papers with proposed prototype have any user study ?
Producing a prototype that solves an interesting problem constitutes a research contribution. Hence, it isn't necessary to conduct a user study. This still holds true when several solutions are known. As more solutions emerge, the original problem becomes less interesting and merely producing another prototype is insufficient to make progress. Hence, it becomes necessary to distinguish new prototypes from previous prototypes. User studies are one way to distinguish new prototypes.