It is a cultural thing (as is living with and maintaining strong ties with extended family), but it's far from a uniquely American thing. Consider that with respect to individualist norms, in many of the most developed countries people choose to take advantage of the option to live separately because they can afford to comfortably do so. People generally don't like dying alone in my experience, but some (in the US/Canada, much of NW Europe, etc.) don't like being suffocated either.
I suspect so. Every inch of our culture pushes 18yo's away from siblings and parents. Some families overcome that and some don't.
Through most of human history, families were physically close enough that parents, grandparents, aunts/uncles became interchangeable & elderly parents didn't have to die alone in nursing homes.