Why isn't the Transition Altitude depicted on SID charts?
FAA

Because it is a fixed value at 18,000 feet.

ICAO-style and Jeppesen

They show the TA (and sometimes the TL) on SID charts.



ICAO Annex 4 confirms the depiction on SID charts (chapter 9):


  9.9.4.1 The components of the established relevant air
  traffic services system shall be shown.
  
  (...)
  
  5) transition altitude/height to the nearest higher 300 m or
  1 000 ft;