Because it has to be done individually for each headphone. It would require specialized equipment, like a super accurate reference microphone capable of accurately listening to a headphone speaker, in the same way as your ear.

The best thing is for the speaker manufacturer to assume zero ohm output impedance from every amplifier, and for them to do their work to get their speaker to have a close to flat response. (Or whatever: some markets prefer a huge bass.) Then it has that designed response in any such amplifier.