A ton of young people leave, because NZ has low wages, very high cost of living, and is very isolating because most places feel suburban/rural and car dominated.
NZ is terrific if you’ve got family wealth to fall back on, but without you’re doomed.
I wanted to stay, but in spite of graduating top of my computer science class, couldn’t find decently paid work.
I moved to Aus 5 years ago and immediately >doubled my salary, could afford a house, and lived in an exciting city instead of a low wage semi rural small town.
Australia of course has many problems of its own, and in particular, treats poor disadvantaged people terribly. But NZ is grim for everybody except the already well off property owning class.