So, I went on Good Old Games and I was really really surprised by the number of games on that service that are published by publishers who otherwise seem to have no problem with Steam, but aren't on Steam.
Just looking only at the 5 star games on GoG:
Blood - Atari, not on Steam
Fallout - Interplay, more expensive on Steam
Fallout 2 - Interplay, more expensive on Steam
Gabriel Knight: Sins of the Fathers - Activision, not on Steam
Heroes of Might and Magic® 2: Gold Edition - Ubisoft, not on Steam
Heroes of Might and Magic® 3: Complete Edition - Ubisoft, not on Steam
Master of Magic - Atari, not on Steam
Master of Orion 1+2 - Atari, not on Steam
Outcast - Atari, not on Steam
Psychonauts - Majesco, on Steam, same price
Tex Murphy: The Pandora Directive - Independently owned by developed, not on Steam
The 4+ star games have a similar ratio. I know Myst and Riven have been on GOG for a long time.
So is this a case of GOG signing exclusivity deals for some of these games, or is it a case of Valve being lacklustre at pursuing classic games? It's just inexplicable that the big guy, who is #1 with a bullet, is missing out on some of this stuff.