Did you not play Lonesome Roads? The backstory is literally the entire reason why you're doing anything at all.
The story only takes place because you happen to be a postman who delivered a bad package, so Ulysses is out to get you.
Maybe RPGs aren't for you, since you don't seem to understand Roleplay.
You're criticising Fallout 3/4 because your minor backstory (being a child/parent respectively) is the basis of your main story.
Whether your character is aware of it at the beginning or not, the backstory of being a courier is the basis of the main story of New Vegas.
And you've completely misunderstood my point.
I'm not saying "Oh, turns out I was always a courier, best continue to be a courier".
I'm saying that you have even less say in why you're on the path you're on (meaning the entire game), because it was all decided in a massive backstory you didn't even get to take part in.
That kills the roleplay choices you can have with your character. Yes, I can be a nice trader, or a bloodthirsty cannibal, or a merc for hire, or the wastelands greatest chef. But at some point, I was always a hard-working courier, who accidentally destroyed a town, and I have been setup by a person who believes I wronged them.
Yes, the Fallout 3/4 characters don't leave too much room in their backstory for you to create your own story as to why you got where you were, either.
As a child in Fallout 3 in a vault, you can hardly say your village was attacked by raiders, or you came to this place as with a caravan, or were hunting an individual.
Nor in Fallout 4 do you have too much room to explain your motivations/personality (Nate has more room than Nora, since he could become bloodthirsty, or even pacifist from being a soldier, compared to Nora just being a lawyer).
But New Vegas is just as bad, in not giving you room to create your own backstory, and then going a whole mile further to explain why you specifically ended up here. Having amnesia, and that letting you being a maniac, is just a cop out. It's easy, cliché writing. And then giving you a big back story anyway is having your cake and eating it too.
Compare Fallout 3/NV/4's backstory to the likes of Oblivion or Skyrim.
All you know in Oblivion is that you ended up in prison. How or why is up to you.
All you know in Skyrim is that you ended up captured by the Imperials, possibly by mistake. How or why is up to you.
They're much better games for roleplay.