Source uses licenced middleware such as Havok so SDKs are likely all we're going to get officially. Source 2 might be a different story if HL3 releases one day since Valve replaced audio and physics with in-house technologies.
replyOld-ish Source has a pretty generous license, no? https://github.com/ValveSoftware/source-sdk-2013/blob/master... GoldSrc is there somewhere as well. I’m definitely grateful for it. I’ve read some of it myself and watched a bunch of Youtube about it.
replyThat repository is not the engine, it's just the code for the client.dll/server.dll binaries that contain game-specific logic and some other tools.
replyWhy should Valve give up their leverage. They actively pick and choose who they license the engine to.
replyRealistically there isn't really any commercial value left in it though. While there were some notable unique games made with it (Titanfall 1 & 2, E.Y.E, The Stanley Parable, etc), these were made over a decade ago and the majority of games made with it are reworks/mods/adaptations of Half-Life related material.
replyhttps://developer.valvesoftware.com/wiki/List_of_third-party...