I think you're missing my point here.
Because in Skyrim you weren't a dragonborn who had to kill Alduin.
You don't have to though. If you never start the main quest, you can go through the rest of the game just fine. It's still perfectly playable and gives you plenty of freedom without adhering you to the main story.
And in FO:NV you weren't a courier who was rescued from the brink of death with a score to settle.
Again, you're capable of ignoring the story if you so choose. And even if you do, every event has so many possibilities and choices for how to proceed. Every junction presents multiple pathways as well as at least 4 different endings and listing what happened to completely optional factions.
And in FO3 you weren't looking for your father after becoming a vault escapee.
Hell, you can just write your dad off as dead and go wander the wasteland if you so choose. There's plenty do and see, and even if you ARE doing the main story to look for your dad, it doesn't dictate what kind of character you are. You can be everything from a saint to an asshole. It doesn't force you to play the character a certain way.
And in Oblivion you weren't a prisoner who became the Hero of Kvatch and had to stop Mehrunes Dagon from causing armageddon via the Oblivion Crisis.
Why were you in prison? When you get out, maybe you don't WANT to give the amulet back. Maybe you decide to hold onto it for a while. If you never give it back, the oblivion gates never open. And even if you do, you have plenty of choice in how to play your character and how you want to develop their personality in your head.
And in Morrowind you weren't a convict who had to help the Nerevarine defeat Dagoth Ur.
Okay, you have to admit that Morrowind is INCREDIBLY free-form and hardly linear in any capacity. That game was made specifically with freedom of player choice in mind.
meanwhile.
In fallout 4, your character will never let you forget they have a missing child. Gotta find Shaun, my baby was taken from my, my wife got killed, better remind everybody even if they aren't part of the main quest. And even if you find quests that aren't involved with the main plot and don't mention Shaun, then you can't help but think "Wait, wasn't I just crying about my missing son 10 minutes ago? And now I'm being all snarky and chipper?"
You're searching for your father in 3, but that detail doesn't dictate who YOUR CHARACTER is as a person. In 4, you're ultimately shoehorned into "concerned parent" and rarely do you get any real options that mold your character.
Name one game that you think isn't linear and I can guarantee you that it is in fact linear in some way or another; games just don't work if they aren't. They need a plot.
In the end, that isn't the problem I'm trying to address. The problem is that plot is written in such a way that it becomes difficult to lose yourself in the roleplaying of that plot, without being constantly pulled into a singular role that you have no choice over unless you just try to avoid dialogue alltogether.
I think it doesn't help that they went for a Shepard-esque voiced protagonist which severely limited the options in speech. Since they can't reasonably record as many lines as previous games in just text.