Because Vietnamese often expresses relationships through forms of address. Kinship terms can mark relative age, generation, and familiarity. The choices a speaker makes can acknowledge a relationship as part of ordinary conversation.
In English you can talk to a grandparent, a boss, a toddler, and a stranger using the same two words: I and you. Vietnamese requires you to choose. That choice encodes generation, relative age, family side, and sometimes marriage.
The effect is cumulative. A child who calls their father's older brother bác and their mother's younger sister dì is being taught the family structure thousands of times without a single lesson. By adulthood, the map is automatic.
It also explains why Vietnamese family vocabulary is so much larger than English family vocabulary. English collapses many distinct relationships into "aunt," "uncle," and "cousin." Vietnamese keeps them apart because the language uses them constantly.
And it explains a common diaspora experience: a Vietnamese American who understands the words but not the system can freeze at a family party, because choosing the wrong title is more visible than choosing the wrong verb.
The grammar is shared, but the specific terms families use differ by region and habit — especially bác, cô, dì, and cậu.