Maybe because from a strictly evolutionary point of view, that’s a failure: they won’t pass on their genes (maybe also culture, values etc.), and other gene pools will take over the resources their lineage worked to secure.
"Always" is overstated. Populations have been reshaped before (e.g. farmers absorbing foragers in Europe, steppe migrations, Arabization of North Africa, the Americas after 1492 etc). So turnover isn’t new, but this mechanism is different. This pattern stems more from our system and choices (schooling, careers, costs, contraception, culture etc) than from violence/war, disease, forced moves, so in that sense it’s self driven, and historically unusual.