Because the offline editor would have to know each firmware difference and be updated with each firmware.
You'd say "today's editor is updated with each firmware too". Yes, but each editor update is only a layout change or parameter add.
With an offline editor, each update would have to handle what the axefx firmware also handles, like parameters transformations when value ranges change,
parameters defaulting when a major upgrade is introduced, creating non-existing parameters when loading an old preset and saving to a newer version,
loading old presets with very different parameters etc... programming nightmare
With client/server architecture used in today's editor, this nightmare is centralized in a unique place : the axefx firmware.