Remember, Romans had a fairly advanced society, and buildings, architecture, and infrastructure. Roads crossing whole regions, (the highways of the time), aqua-ducts, ways to dispose sewage, etc...
This all slowly degraded and eventually disappeared with the fall of the empire.
Stone building were replaced by mud huts, and I don't know about you, but not having running water in your town, or paved roads to the next town or port seem like huge drawbacks to life quality.
The first Anglo-saxons that came in Britania, after the Roman left lived in what you would call downright primitive huts made by either mud or wooden planks with mud in them.
That and the constant warring, in many ways, it was a huge set-back for the people there.
London in the "Dark Ages" didn't have aqueducts. But Londinium was part of the periphery of the empire, and it also never had aqueducts. There's not that much information to be gained by cherry picking the elements of the central core of the Roman Empire and asking why they weren't present in far flung areas in the post-Roman era.
Skeletons show that people were relatively healthier (at least in the periphery) after Rome had fallen than when Rome had dominated these regions.
After a quick search I would say the latter is more important here: what we observe from the bones are things like a quality of nutrition (e.g. vitamin deficiencies) or effects of some illnesses. Inadequate nutrition signals poor living condition and illnesses affect even the most "gifted" individuals (don't forget that hardship selects also for other traits than just a good immunity).
In Austria and Germany most people descripe the Nazi era also as “dark times”, despite there beeing roads built (although the Nazis were so in debt at the beginning of the war already that you can hardly call this a success).
So dark has multiple definitions. I thought the most.common one was about a time where rational thought and humanism didn’t really win, but tribal rivalry (on any scale) and radical beliefs did
The issue is that the Enlightenment wasn't a project to move human beings to some better plane, but to rationalize society and its members in a way that made it more amendable to control and planning. And so centralized power could use Enlightenment ideology to eliminate alternative power centers, centralizing power in the hands of a single state that could reorganize society for its own ends.
That itself had pros and cons, with the biggest con being the newfound ability for states to execute projects of mass violence more effectively.
I read somewhere that skeletal remains show people got taller after the end of the Roman empire. And then started getting shorter through the Renaissance right up till the agricultural revolution in the early 20th century.
The Early Middle Ages saw relatively little
population growth with urbanization well below
its Roman peak...Estimates of the total population
of Europe are speculative, but at the time of
Charlemagne it is thought to have been between 25
and 30 million
BULLSHIT. How on earth you can present blatantly false presumptions as information?
https://en.wikipedia.org/wiki/Pre-Romanesque_art_and_archite...
"In most of western Europe, the Roman architectural tradition survived the collapse of the empire. The Merovingians (Franks) continued to build large stone buildings like monastery churches and palaces."
BTW we still have Roman aqueducts, roads and bridges all over Europe.
Mentioning that there were plenty of building and construction happening in some part, doesn't negate that the rest (80%) was being ravaged and in continuous decline to the point of whole populations being displaced.
Only the Eastern Roman Empire (aka. Byzantine empire), had a unified government structure, yet it was struggling with the Slavic invasions/incursions from the north, and the wars with the Sassanid empire and the succeeding caliphates.
They did attempt to re-unify the empire with the Justinian Restoration (with Belisarious being the main general re-conquering vast territories), but that didn't last long either. As the economy was lagging the region was ravaged by the Black Plague, where it is estimated that 25–50 million people in two centuries of recurrence died, equivalent to 13–26% of the world's population at the time
https://en.wikipedia.org/wiki/Plague_of_Justinian
To me, late Antiquity end, and the true dark ages begin after the death Justinian, and the decline of the Eastern Roman Empire to the point that it couldn't defend or keep its territories from the Slavic invasions.
Within that context it's hard to believe that the European tribes simply forgot how to build roads or monuments or architecture after Rome ceased to influence them.