About the character already defined, [..]
Lets start off with little
background: Planescape Torment, considered to be one of the best cRPGs, where the character has an extensive predefined background. While alpha protocol --another RPG title Chris avellon, obsidian, worked on-- has several predefined backgrounds (e.g. in one of them you had several PhDs).
As for your generalizations/examples, they suffers from confirmation bias.. Yes FO:NV story structure provided the most blank slate character, however, the single most notable difference between rest is the "engine", not structure. FPP games provide a more detailed visual environment, if you take the time to reconcile those difference with what we know about FO1, you should be able easily match your initial comment of what's wrong with FO4.. In fact I did so for FO2 --my favorite title-- and compared its intro with FO3 --my least favorite title-- and decided that FO2 intro is completely sterile and illogical..
Also you seem to favor the classic fantasy setting i.e. teenager, orphan, who is unwillingly trust into an epic journey.. hence you consider a child a limiting factor to your character build.. never considering that not having one can be as limiting to others ... (I am not interested in bachelor perk, nor sexual experimentation in games)
Lastly i'll leave this video here, which should present a different pov on story structure, and hopefully answer your initial question "What were they thinking?" (it mentions FO:NV a lot )
People reacted with pitchforks and torches to Dante getting black hair, they yelled Betrayal at Xcom FPS, they hated every single thing about Resident Evil 6 and the new Alone in the Dark game, But then they turn back and tell people to "accept changes" on Fallout. Why?
Toby ehrm Walpknut, god no, please NO!!! Fallout 3 came in 2008, its 2015 ! and you are still here every single day ranting about Beth...