They basically have. Half Life has been Open Source for a while, Portal and Half Life 2 were both more-or-less open along with the Source SDK. Team Fortress 2 a

They basically have. Half Life has been Open Source for a while, Portal and Half Life 2 were both more-or-less open along with the Source SDK. Team Fortress 2 and CS:GO aren't technically open, but their source does exist in the wild and can even be built on modern machines. That still leaves DOTA, Artifact and Half Life: Alyx without any proper source code, but the stuff people actually care about has mostly been freed.