Professionals move from city to city all the time due to jobs. In the modern world it's easier to keep in touch with family and friends with Internet and cheap (by historical standards) flights.
I have 3 close friends - they live about an hour away, about 8 hours drive away (or about an hour of flying + 1-2 hours driving), and one on the other side of the country.
"Keeping in touch via Internet" for me is a pale imitation of what I want from a friendship. We text a few times a week. I visit each of them 1-2 times a year. I miss them a lot.
This isn't to claim it's impossible to do better. But I have a feeling that especially those on the "urban" side of the friendship just have a hard time making time for say, a Zoom call with you on the same frequency as they may have gone out with you for dinner or drinks. Especially group dynamics don't allow this: If you were part of a group of 5 friends that hung out together, they'll just start hanging out without you, and no time will be opening up in all of their schedules to devote to tele-friendship. Those friendships will suffer.
And remember, the "low cost of living, low earning" thesis we're discussing does mostly rule out even those "cheap" flights (which stop being cheap really quick once you have a family, so multiply all prices by 2, 3, 4...)
