Because family terms offer a familiar way to express relative age and social relationship. Speakers often choose a title such as older brother, aunt, or uncle even for someone outside the family. Other forms of address exist, and usage depends on context.
In everyday Vietnamese, you address someone with a kinship term and refer to yourself with the matching one. Talking to a man a few years older, you say anh and call yourself em. Talking to a woman around your mother's age, you might say cô or dì and call yourself con. The pronoun changes depending on who is in the room.
This means every conversation starts with a small, mostly unconscious calculation: how old is this person, roughly what generation are they, and where does that put me?
It is not pretending to be related. It is a way of showing that you have placed the person respectfully. Using the wrong term — calling someone em who expects chị, for example — can land badly, which is why people often ask "chị bao nhiêu tuổi?" surprisingly early in a conversation. The age question is grammar, not nosiness.
There are neutral-ish options. Bạn (friend) works for peers, and formal writing has other conventions. But in speech, the family words do most of the work.
Usage varies a lot by region. Northern and southern speakers apply bác, cô, and dì differently, and younger urban speakers use bạn more freely with peers. If you are unsure, listening first is genuinely the best strategy.