We can also just look at the reasons people today still live in densely populated cities, despite big drawbacks such as the cost of housing. Proximity to jobs, universities, recreation options, grocery stores, and more. It makes perfect sense that during COVID when much of these benefits were essentially eliminated due to lockdowns, that the negatives began to outweigh the positives for many people, and so they moved away.

I don't know exactly what your idea of an anarchist utopia looks like, but if it still involves things like universities, a wide variety of available jobs, various recreational activities, etc. then I don't see why the desire for people to live in cities would change?

Also, not really related to my main point, but still: Yes throughout history we generally lived in rural communities, but this was not due to desire but necessity. For most of history small areas simply could not sustain large numbers of people, not too mention the other problems like housing and disease. But once we worked out how to sustain ourselves, we started living in larger and larger groups. It just so happens that some of these problems were solved under capitalism.

I feel that this is relevant to the discussion:

https://en.wikipedia.org/wiki/Behavioral_sink

Behavioral sink is a term invented by ethologist John B. Calhoun to describe a collapse in behavior that can result from overpopulation.

Many [female rats] were unable to carry the pregnancy to full term or to survive delivery of their litters if they did. An even greater number, after successfully giving birth, fell short in their maternal functions. Among the males the behavior disturbances ranged from sexual deviation to cannibalism and from frenetic overactivity to a pathological withdrawal from which individuals would emerge to eat, drink and move about only when other members of the community were asleep.

Having reached a level of high population density, the mice began exhibiting a variety of abnormal, often destructive, behaviors including refusal to engage in courtship, and females abandoning their young. By the 600th day, the population was on its way to extinction. Though physically able to reproduce, the mice had lost the social skills required to mate.

"Calhoun's work was not simply about density in a physical sense, as number of individuals-per-square-unit-area, but was about degrees of social interaction."

Obviously rodent studies are only so applicable to humans, but I see myself and worrying modern societal tends in some of their behavior and the ways they suffer.

I think that when we interact with too many strangers every day that we're unable to make meaningful connections with any of them, leading to stress and illness. If we had few enough encounters that we could come to recognize most of them, it would build trust and a sense of community.