The portal gun doesn’t really fit in a Half-Life game. The mechanics of the gun almost demand an enclosed space, with flat surfaces and puzzles that require the player to understand that they’re solving a puzzle. The portal gun would break the outside world too easily, as players figure out how to just zoom past everything, and not follow the linear path that FPSs like Half-Life guide towards. Testing surfaces for game breaks and boundary checks would be a QA nightmare. It doesn’t kill enemies in any useful way, which is the primary function of a FPS weapon.

It is a puzzle gun, in a puzzle game. And that’s okay.