Simple answer, because speakers to be equal to headphones would need to be sealed to your head! In otherwords the outside environment your speakers are running in is over 50% of the sound results which is just as important as the speakers themselves... Headphones are not in a room, hell they are not even really capable of making too many mistakes as they are 1" from your eardrum.
Its similar to sitting in a car with a good stereo system, its almost a sealed speaker all in itself, the evironment is much more controlled.