Chief talks, just not a lot. Samus also talks in Other M and has written internal dialog in the 2D games.

If I remember, I think Chief started talking more in Halo 4 afterwards

Doomguy forgot to mute himself while he was busy wallhumping for secrets

Gordon doesn't need to hear all this, he's a highly trained professional.

He's too antisocial, and the Combine censor discord heavily anyways.

He joined a Matrix server but never logs in