Certain apps and the start screen do not rotate, however other apps and core apps do rotate - that functionality needs to be added by the developer.
As there is nothing on the o/s level that will render what is displayed in landscape - it's possible but not really feasible as it would need to take into account an insane amount of variables.
Plus not every app is the same or follows the exact same design in terms of layout etc. Therefore the developer needs to add landscape support into their app so they can place the UX elements exactly where they want thus tailoring the user experience to their liking.