Because bethesda wanted you to be a part of the game, or some shit like that.
With any Bethesda game, it's the story and the narrative that shapes the character (aka the player) and not the player that is pushing the story. I havn't played Fallout 4, but from what I have seen so far, it looks like they follow the same footsteps as Fallout 3. Just worse.
Here is what I mean with that. In Fallout 3 the player was actually not the main character of the game. This sounds a bit strange, but think about it how often you carry out the agenda of someone else and how little oportunity the game gives you to move out of the tight narrative. At no point does the game actually give you a chance to have some motivation on your own, or to to define some goals on your own. What I mean isn't even just choices and consequences. No, I mean alternatives in the narrative, actuall conversations with the characters where you can epxress the motivations of your character. In the case of Fallout 3, a conversation with some NPCs for example where you explain in detail why YOU care about the purifier or why YOU want to find your father etc. Even a simple response like,
the fuck do I care about this? as long I can get the chance kill something! or
I really believe in the improvement of the wasteland because (...), would be appropiate. Both Fallout 1 and Fallout 2 gave you a lot of oportunities to just do that. Some characters had no other purpose but to get the player in a conversation.
Great role playing hasn't always to mean to just do good vs. evil choices or any choices at all, it comes from good writing, giving the player an array of answers that he can chose from.
An example, for the case you remember it. In Fallout 2 you have a situation where the NPC is tasked with the investigation of the death of one of the crime-gangs in New Reno who died on an overdose of Jet. At some point the character can ask a local drug dealer about it. You get a lot of choices to pick your answers to the character. It doesn't change much at the outcome, but it gives you a chance to express what kind of personality your character has! In Fallout 3 your character has the personality of a log and in Fallout 4 it's always that angry/grieving parent looking for his/her child.
You might think, hey! But we're searching for our father/son, right? That's a big motivation! Sure, in the begining yeah. But when you think about it, what about those that don't care about it? Or what happens once you find your target? Your father is actually the REAL protagonist of Fallout 3. And this is very bad for an role playing game where you're supposed to actually play the game. I am not against tight narratives. Planescape torment for example has a very tight narrative but it gives you many oportunities to set your own goals within the frame work of the game.
Considering how often Bethesda mentions that you have totall freedom to do what ever you want, their games are extremely restricted trough the narrative and quest design. Sure, you have the freedom to venture in any direction, from one corner of the map to the other and to kill anyone that isn't
essential with any weapon of your choice. But is that real freedom? Is Call of duty less of a straight forward shooter beacuse you can chose to use a pistol instead of a sniper rifle to kill someone? In the end, it doesn't mean anything to the game, because the narrative never ever gives you any kind of freedom. Does it really matter if you move in Fallout 4 to the east instead of north when you leave the starting location? Why is the fact that you can raid now a ruin instead of a cave important when it has no meaning to the plot? Or that you can kill thousands of bandits, raiders, mutants, giant spiders or what ever if they add nothing to the narrative. Not even in those rare cases where you actually have two choices to solve a quest do you have a chance to role play anything else but what the designers force on you. If you offer someone two rail roads that lead you in the exact same direction, it's still following the same course.
And when you look at the dialog of Fallout 4, it's even worse then Fallout 3. Almost all the answers mean the exact same. When ever someone asks you to do something for them all you can answer really is, Yes, Yes but later, Yes (sarcastic). And in the situations where you actually have a conversation with NPCs, you're ALWAYS forced to answer in a very specific way. Which is even made worse that you have a voiced protagonist.