http://en.wikipedia.org/wiki/Auditory_system has the basic statement. Your outer ear lobe (the pinna) is shaped and convoluted in such a way that sound is differentially attenuated depending on its point of origin.Your brain's auditory system can make use of the different frequency spectra of the signals from each ear, as well as the overall amplitude and phase differences, to perform three dimensional localization.Unfortunately, when you use headphones, the sound source is directly against your ear lobes and directed straight into your ear canal. There's no opportunity for the pinnae do to their normal job of attenuation and distortion necessary for three-dimensional auditory response. There does exist signal-processing software which can apply the "human average" attenuation response to the signals going to each headphone. This can provide a simulated "surround sound" experience to the listener. I found a decent article from Dolby Labs via Google.On a side note, unlike most mammals, humans do not have the ability to move or rotate their pinnae. That ability provides far more precise localization for other mammals.