The simple answer is a Better Health and Stable financial position
In Developing economies where a general household is low-income, older parents are seen as the responsibility of the children.
With poor medical facilities and a non-existent government pension program, most people aged 60+ cannot work, have low income and suffer from diseases.
In such a case, their children take care of them and live together in a house.
In the Western world, adults earn better have have better medical facilities, which leads to more Independence, freedom and non-reliance on the family.