The only weapon in the second game that could reasonably by used to shoot long distances (the crossbow) already had a zoom function. Everything else was either too innaccurate (submachine gun and pulse rifle) or too short ranged (Gravity gun, pistol, magnum, typical FPS shotgun, and crowbar, unfortunately). There's also the RPG, but that makes up for it by virtue of the fact that you can manually aim it. Simply put, the games don't need it.
Edit: Also, take into consideration that, for the console version, the current location of the zoom function is uncomfortable to use for standard ironsighting, and moving it to to the right trigger, as is typical, would mean displacing the "secondary fire" button. I'm not going to play the game if I can't fire plasma orbs and bullets at the same time.