[realslimjd]

Half-Life, Counter-Strike, Portal, and their sequels are all Steam exclusive. The only mandated Steam exclusives are Valve games. Anything else is purely out of convenience for the developer.

[bsammon]

I realize you're probably referring to PC gaming specifically here, but it's interesting to note that Portal and Portal 2, at least, (haven't checked on the others) are available on the Nintendo Switch store.

[nightski]

Yeah I considered mentioning those but they are also on consoles.