Instead we have social mobility so that we can move anytime when better opportunities arise. That may be a good or a bad thing depending on which side you are.
Moving every time a better opportunity comes up is a privilege enjoyed by a small section of the population. The vast majority of people move rarely, and when they do move it's usually within the same locality.
Even for people who can, family (spouse, kids in school, nearby relatives, etc.), friends, and so forth tend to make moving cities a pretty significant decision at some point.
Th trend line has been toward significantly less mobility relative to past decades--though you'd probably want to correct the numbers for demographics given that 20 somethings move the most (as one would expect).
Exactly-- the number of young people still living at home has surged in the last decade. This is not surprising given that "starter homes" are one of the main targets of housing speculators. People who would buy their first house rent longer, driving up rental rates and putting the transition from family to independence further out of reach for many.
Without home ownership you are throwing away a large portion of the money you may make in your life. Even if you move frequently it still makes sense to buy so long as you’re not hitting the tip of the market.