Why are first and second person pronouns?

The first and second person pronouns take the place of the nouns for the speaker(s) and the person(s) spoken to.

For example, you wouldn't say to your sister: "Jack made a sandwich for sister."

You would say: "I made a sandwich for you."

See how clumsy it is if I didn't use the pronouns in my answer:

"For example, questioner wouldn't say to questioner's sister"

Language develops from the ways in which people use it. People found it easier to use the shorter pronouns than keep repeating the nouns, which can have multiple syllables (my three nephews all have three syllable names, using 'you' is more efficient).