Why say hello?

Here follows the wisdom of sdcv, abridged and with emphasis added:

$ sdcv hello

-->Webster's 1913 Dictionary
-->Hello

(interj. & n.) See Halloo.

$ sdcv halloo

Found 2 items, similar to halloo.
-->WordNet
-->halloo

halloo
     n : a shout to attract attention; "he gave a great halloo but no
         one heard him"
     v 1: urge on with shouts; "halloo the dogs in a hunt"
     2: shout `halloo', as when greeting someone or attracting
        attention

-->Webster's 1913 Dictionary
-->Halloo

(n.) A loud exclamation; a call to invite attention or 
to incite a person or an animal; a shout.

Hello!   Halloo!    Hello!