Why isn't the Transition Altitude depicted on SID charts?
I would assume it would be a relevant information to be presented on this type of chart.
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):
220.127.116.11 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;
It is shown, when necessary.
But some SID procedures may not go high enough to be worried about TA. eg if a US SID only went out to say 10,000ft, then they wouldn't need TA on the plate.
Europeans tend to have a lower TA than the US, so the tendency is for the SID to climb through TA, and so it needs to be on the plate.