If notation could hold all the levels of detail that can be expressed in MIDI, they would probably be too cluttered, or have too many symbols to remember. After all, the few level symbols (ppp pp p mp mf f ff fff) don’t really cover the 127 of MIDI. Or we would have to depart from the traditional symbols and go for things like L54 or L114.
I remember one particular MIDI rendering of a recording of ‘Somewhere Over the Rainbow’ that was far too complex, and contained many shifts in tempo, indicating that to be notationally defining it specifically enough to ensure it is reproduced exactly would take far too much time to encode.