One reason is that you have kids and you want to have more space. Also, your cost of living goes up because you need more resources for a larger family. You might spend more on education, food, vacations, and so want to reduce rents or mortgages. You can do all that stuff in any big city, many do. But kids are common reason to move out of a city center.

I would agree in principle, but from experience it seems people are less interested in the energy of the city as they age and acquire more things, and prioritize building their own space around them instead of utilizing the city around them.

Many of those amenities are not as relevant when you're older. Easy access to bars, restaurants, and clubs is not as important as a larger home and good school districts when you're a parent, for example. Access to things like healthcare, grocery stores, and target are not any more difficult and often easier to access in suburbs.

Basically, people's priorities change and the optimal living situation changes accordingly. On average, that is, I'm sure some people prefer city life into later adulthood, and some young people like suburbs.

Kids. You will value space much more than you value proximity to restaurants.

Also, the amenities available in a city are quite limited as you age out of your 20s. Food and necessities become a much smaller share of what you are purchasing. Eventually you want access to tools and hobby supplies and home goods - stuff that is logistically not hauled into big cities but dropped off in the suburbs.

Because the amenities you value change over time. There’s very little I want or need in the city any more. Plus these days, lots of amenities will come to you.

If the car companies manage to get level 5 self driving working well, I’ll move even further out of the city.

People don't value all amenities equally, and as people age, their personal amenity-valuations change, often in stepwise ways. For example, prior to having a school-aged child, an individual might value good-elementary-school-access very little, but if they have a child, their valuation of that amenity will likely rise sharply within a few time-needed-to-move units of their kid being school-aged.

I think some people are more comfortable with death than changing priorities.

To add to your point: The way we push people into the suburbs in the US leads to more loneliness for adults. If I want to meet up with friends, we usually all have to drive, regardless where we meet because nobody has a pub or restaurant or place where we could do any activity together near their homes. The only exception is one friend who lives in the city proper, but he lives across town, so everyone else has to drive extra far. In contrast, my friends in European cities seem quite happy living in the city proper, sometimes with kids, and they'll just meet with friends and family for brunch on the weekends and can walk or take public transit.