Because they're not Steampipe'd yet
I don't know if Valve will actually put that on Steampipe
Which saddens me, because the maps in Gmod were so very close to being a near perfect recreation of Half Life Source in a different game
I remember when they were still being worked on, there were no NPCs, i think they used the Half Life 2 world models for weapons like the shotgun and such and any HL1 specific weapons weren't there, cubemaps were messed up, and events and levels wouldn't trigger properly