Financial security is the reason - if people don't have financial security, they'll expect their children to care for them in their old age when they're unable to earn.
That's the case in most (all?) immigrants cultures - children are expected to, and do, take care of their parents in their old age (exceptions are always there ofc).
After families are established and have financial security, they no longer need to rely on their children in their old age - at which point it becomes a choice.
After families are established and have financial security, they no longer need to rely on their children in their old age - at which point it becomes a choice.