Being an island that gets all its goods through cargo ships or planes allows a lockdown as strict as NZ’s. That cargo influx involves a small number of personnel. However, that isn’t the case in e.g. Europe, where countries have not been self-sufficient in foodstuffs etc. for long decades, and many people live on one side of a border and work on the other side. That means there is a steady flow of trucks across the continent, as well as commuting of at least workers judged essential.