1. Because in real life, sound doesn't emanate 1/2 inch away from your ear - unless its a mosquito. So its easier to approximate live instruments using speakers in a room.
2. Because music is mixed and produced using speakers in a room as a reference. That is a radically different listening environment than headphones. You might as well ask, if I listen to music underwater, why does it need a special EQ curve to sound right? Because you are frickin underwater! The music was produced to sound good in air not water. Likewise, the music was produced to sound good with flat(ish) speakers in a room. Not flat headphones with the music blasting straight into your ear canal.
The radical headphone curves exist to compensate for this radical difference. The intent is to, as much as possible, mimic a good speaker in a good room, when in fact the sound is blasting straight into your ear.
The OP's suggestion would be a legit solution though if you could work through all the legacy issues.