Why not midi? As a musician, I'll ask you a parallel question: Why not write all documents in flat text? Who needs bold, underline, different fonts, pagination etc??
Midi doesn't make pretty sheet music. It only notates "note on" or "note off" or "patch change." It doesn't even notate which score a note should be on. This means, for example, that piano music would be just about impossible to play from a raw midi dump.
An XML based markup, or the TeX based Lilypond allow for pretty, easy to read, scores.
On the other hand, Lilypond has a midi import feature, so MIDI IS useful. It just requires some editing to make it human playable.