AFAIK, because QNE refers to the reading on the altimeter when it is on the threshold of the runway in question. So: it is not an altimeter setting like QNH or QFE; nor is it the reading of the altimeter with 1013.2hPa/29.92inHg set (that is 'Pressure Altitude'); rather, it is specific to a runway threshold and enables a pilot to add the desired circuit or procedural height, thus yielding the desired altitude at the time.
I think?