You do not have to hang dry. Hanging has a couple positives that may seem minor are still worth considering but in general it is just the most convenient way to do it for a list of reasons.
 
Unless the temperature/humidity levels are way off it allows for a slow dry where all the non-psychoactive THV would turn psychoactive and all other chemicals that need to be broken down to make sure your harvest is the best it can be in every way have a chance to break down.
 
Its simplicity and flexibility makes it easy for almost anyone to be able to find a location where they can provide an adequate amount of slow but constant air circulation and at least to some degree regulate temperature and hopefully to some degree humidity also, but hanging usually makes all that easier to do plus the results are better and more even when hung.
 
It is a case of it has long been recognized as the overall best way to dry for most people in most situations, though at times some reasons for its use were based in inaccuracies, so it has become the standard practice.