Non-Broken Steel Fallout 3's ending is shit so I have no argument with you there.
History is a more chaotic organism than that as while humanity has bounced back from numerous disasters, we also have quite impressive gaps in terms of discovering and losing technological as well as social advantages. Aristotle, for example, knew how octopus reproduce and had a surprisingly capable zoological knowledge which was lost or ignored until the 19th century.
High Islamic culture was effectively destroyed by Tamerlane and his Mongol Horde with rebuilding eventually occurring but taking centuries. Carthage never recovered and Libyan civilization took a dramatically different turn. While the "Dark Ages" is a misnomer, it's interesting to look at Rome specifically as a historical location and you get something which actually looks like Fallout in some ways as people used the same aqueducts and locations built until they crumbled to uselessness with no ability to rebuild them (or more precisely, no funds).
Also, the Rule of Law is the exception rather than the rule in most parts of the world. Feudalism was a protection racket which existed on the social contract of, "I'd rather give up X amount of my crops regularly to these guys than Y amounts to an unknown someone else."
Even so, the Slavic People invited the Vikings to rule them in large part because they hadn't been able to set up their own military defense and system of government. And there are regions which remain chaotic in Africa due to colonialism and interested foreign powers despite a century of departure. How much worse is it with wholesale environmental destruction brought out by, well, fallout?
All the examples you give have a common point : they are all nations which got defeated by another, and the victorious had time and resources to insure that the loser couldn't rebuild its former identity. In which case, sure, rebuilding can be difficult. I mean, look at Constantinople. But this is a very specific situation, and it is not the case with most of the disasters that can blow a civilization away. I could oppose the aftermath of the black plague, for example. Despite causing more deaths than a freaking nuclear war, it only took fifteen years for the banking services and the agricultural conglomerates to rise. Or how fast we rebuilt Stalingrad or Hiroshima. Or hell, how fast Germany was back on its feet, despite what happened to it in the last months of the war.
In most cases, humans have a natural tendency to help each other, rebuild fast, and establish the rule of law, wether it is brutal or civilized. We are naturally cooperative, even for selfish reasons.
The thing is : If humans naturally become murderous sociopaths in the absence of modern society, then
how did modern society ever form in the first place?
In crises like Atlanta's blizzard-induced traffic gridlock, Hurricane Sandy, the terrorist attacks in Paris, examples of basic human kindness weren't difficult to find. When tragedy hits a country, most people's first impulse is to see what they can to do help their folks, rather than to carry all of their canned food into the basement and start loading their rifles. It's not even because we're nice guys -- it's because instinctively we know that we might need that person at some point. So even if some worldwide crisis were to transform us all into selfish mutants, the reciprocal altruism suggests that we'd probably still be willing to share that can of beanie weenies if it meant we could get something in return later, even if that something is just "an extra person I can feed to the ghouls to make my getaway." That's the biggest thing missing from Bethesda, or hell, from "the road" and from "the walking dead". They shouldn't have to scavenge in vacant houses for food or medicine; somebody should be going camp to camp selling that shit in exchange for bullets, sex, protection, or whatever. Hell, even monkeys figure that out in less than a week. No, really.
"This war of mine", "Fallout 1-2" and "the last of us" and even "Dishonored" (where half the population dies of plague in a matter of months) dealt with this realistically, and the obvious result is immersion on steroids. None of these titles show a world made of chocolate and boobs, mind me. It is brutal, unforgiving. Yet, it works. I don't see a single reason why the commonwealth wouldn't have have a functional system after 50 years, even less after two freaking centuries. At this point, there should be city states, thousands of citizen living around Boston, militias patrolling the streets, production sites and an agricultural system.
Unless the Institute prevents it, alright, and that's a solid reason. Yet, the Institute only started to mess with the commonwealth this century. So what did the citizen do for the first century after the war ? Hunt squirels in the forest and raid, like cavemen ? Why ? There was nothing to prevent them from rebuilding.
As for the rule of law, you mention Feudalism, as an example of a civilization where the rule of law didn't exist. The thing is, feudalism is the perfect example of a functional rule of law. Sure, you had to pay some sort of tax in exchange for public service and protection. But considering that taxes on revenues didn't exist, that peasants worked less than we do and had more free-time than we do (one third of the year, more exactly), the system is not that different from ours. Hell, you still pay a fee for everything you buy today, and "fee" is derived from "fief". I live in Alsace, France, where there are tons of wine producers. Many of them have been there since the 13th century, and have archives of their sales. And guess what, even when the lord was overthrown by an opponent, there wasn't any change in production for them. Because peasants' production wasn't affected by who they served.
So, sure, you had bandits (usually knights. The irony.) and wars. But first of all, medieval wars were rarely "deadly" for the common folk, as battles were extremely rare (and since you ransoming your enemies was the main motivation, killing them would be highly improductive) and banditry is not incompatible with a functional state, government, private sector and banking services. There is banditry is Somalia today, yet Somalia has universities, factories, a government, a police and an army.
As for the roman knowledge, sure some of it was lost. But there's a reason why monks started to copy their texts only decades after the fall of the Empire. And let's not forget that Byzantium kept all the roman knowledge for centuries (hell, Sainte Sophie was built under Justinian's rule, and it only took 8 years to build), and kept it to itself until its destruction. At which point, it finally exported to Europe and provoked the Renaissance.
