Question

Why do we say hello?

Answer 1

Hello is a salutation or greeting in the English language. It is first attested in writing from 1826.

Answer 2

heya,

good question!
why do we say hello?
when we meet with somebody to start talking we say hi, hello, heya, hey.
after that we continue our talking or chatting. this word is used when thomas edison urged the people who used phone to say the word "hello" while answering. 
thank you.