One thing about video games that I find a tad bit annoying is when your choices in the game don't really have an effect on the outcome of the game. Now it may change a few minor details, but ultimately the game ends the same way. But that doesn’t mean I like them any less, in fact, some of these are my favorite games. In Dragon Age, you can do the Dark Ritual in Origins or skip it but either way you end up killing the Archdemon. And yes, one allows the warden to live but the dragon is still dead. In Mass Effect, you can save the council or let them die, but either way you stop the reapers from destroying the Citadel. In Until Dawn, you can choose what to do in key situations and the only thing that happens is relationship change/someone may die but the game still ends the same way, the wendigoes are destroyed.
So why make choices when it doesn't ultimately change the plot line?
Simply to put it, the choices are more of for the player. The plot is going to stay the way it is, but the choices allow the player to feel more in control of the game and help for submersion. Now in the examples I gave earlier, though the plot is static your choices do have consequences on the characters and how they interact in the game or sequels.
So why do the game developers have static plots?
When they are building the game, creating a hundred different plots based off your decisions is hard. You have to change cut scenes for every situation and if it's in a series like Dragon Age or Mass Effect, that makes it impossible to connect all the games together if everyone playing the game has a completely different ending. The follow up game would only make sense to a handful of users that made those same choices. So creating a game that has a bunch of endings makes it impossible to follow up, but if player choices only effect character attitudes then that's a lot easier to deal with. Is the character diplomatic? Yes, then have them act like this in the next game. Or have them act in a different way. But those attitudes, mean very little to the plot. They just change how players feel about those characters.