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.

> Why don't sources try to measure the frequency response of the driver + cable + headphone circuit and apply a digital prefiltering step to compensate these distortions?

Because the effects are negligible and trivial to overcome with even basic attention to design.

The distortion of speakers or headphones is orders of magnitude higher than the distortion from even a basic headphone amp design in the modern era.

You don’t generally want to compensate, as impedance that isn’t flat doesn’t necessarily mean that the output SPF isn’t flat.