Because that is how we understand eachother initially, it being a bit of an effort to hold several billion completely distinct identities in your mind at once. Especially as we are only an evolutionary blink away from the time when we would only ever have known a handful of people for all our lives.

Hence it is a useful shorthand to be able to lump people into such categories as Technician, Ethiopian, Teenager etc. etc.

We can then even start to elaborate this into combinations, such as Ethiopian Technician and so on.

Once you know someone well these categories probably become far less important than personal knowledge of the person.

Such categories, of course, also allow one to identify with other people with whom one shares attributes/skills/interests. Such as Modern Antiquarians. It's not everything we are, but it describes a part of the whole.

Why do you think we give ourselves labels Postman?