Why So Many People Age 25-54 Not Stay in SD?

It seems like a revolving door of new people in that age bracket always moving in, but then never stay and always seem to move out? Yet the longtime residents seem to be those age 55 and older and they seemed to have moved here years ago and never left.

Why is SD so transient when it comes to people in the age 25-54 bracket? It makes it hard to really establish any friendships or relationships.

It seems the single people move to other cities like Chicago, SF, Seattle, LA, New York City, etc...and the people that are not single seem to move to other parts of the country.