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.

They are in no way open source. In fact Valve has killed some projects.

They've released five games in the past 4 years[0] and have another one in active closed testing[1], along with keeping DOTA2 running with major game changes and mini-games[2].

[0] https://en.wikipedia.org/wiki/List_of_Valve_games

[1] Random article about it: https://gamerant.com/deadlock-gameplay-footage-valve-leak-6v... .

[2] DOTA's had massive patches recently, including adding a new attribute, expanding the map, and redoing every hero to have innate abilities and a choice of passive effects. They've also been adding some pretty big PvE/external content events between Aghanim's Labyrinth and the current Crownfall battle pass (which just added a surprisingly fleshed out fighting game mini-game).

Dota 2 is a prime example of enshifitication of modern games where content is added for the sake of content and regards for original style are completely ignored.

Ironically, Dota 2 would benefit as a game if they STOPPED developing it and focused just on stability of the game, but alas.