If I had to guess, it would be because that's not what people want from a Terminator game.
The Terminator franchise has always been about fighting back, the war with the machines. Yes, most of the movie is spent with the heroes running away, but they are still fighting. They're still unloading entire magazines and clips into the Terminator chasing them.
In the future, it's always about the war. The humans have already learned to fight back, and that's what people want to play. They want the epic war with the laser guns. When I was growing up, my friends and I all LOVED the arcade light gun game because the first few levels were set in that future. I'm still hoping for a movie set entirely in that future. The sense of desperation in all the battle scenes would be epic.
To the best of my knowledge, only once has what your asking for happened in the movies. In Salvation, Reese as a kid is shown running and hiding from the machines. And it was boring. That's not what the franchise is about.
Now, could this be done? Certainly. The idea of evading the machines, hiding in sewers and fighting as a last resort works. Alien: Isolation did that very well I think. But eventually, I think people who pick up a Terminator game are going to want to fight back. They're going to want to be able to triumph over the machines. They are going to want their own laser gun, because that's what they expect from the series.
