I don't think anyone is implying that you as an individual flying causes your family to spread out. although if you spend most of the year traveling, your relatives might start to wonder what the point of living near your primary residence is. I can see how you might interpret the wording from the original post as saying that, but it doesn't really make sense that way.

the point is that, when it's affordable and normal to fly great distances on a yearly/monthly basis, it causes people to spread out a lot more. then you end up with a chicken-egg scenario where people "need" cheap flights to see their spread out families, but the cheap flights also enable them to spread out further while still seeing each other.

I think it's not that simple. Flying to see your family is always more costly than being close to your family, even when flights are "cheap". What causes people to want to move? What causes people to be ok paying the cost?

choosing where to live is always a tradeoff between many competing factors: proximity to family, intrinsic appeal of a particular area, job opportunities for you and your partner, the list goes on.

odds are, you didn't happen to be born in the optimal location for your desired career. if you can get paid $20k more on the other side of the country and a flight home costs $200, you might find it pretty tempting to move. if the flight cost $2000 or you had to drive for three days to get home, it would be a lot less appealing for people who value seeing their family.

I don't understand how it's so hard to see that cheap flights make a big difference at the margin.

You are implying that there are a lot of people that are on those margins. There's simply no evidence of that being the case.

In fact - the drastic rise of living costs in economic centers indicates that people will take that move, even if that trip back is several hundred dollars and takes many hours.