Because it’s nice? Not the answer you are looking for, but still I think it is.

It originated as a way of attracting someone’s attention. “Hi” is probably a 19th century American variant of Middle English “hy” or “hey” (source)
Nowadays, it functions less to call attention and more as an informal way of signalling to someone that you’re meeting on friendly terms. It’s the verbal equivalent of a little smile.

i reckon @Harp has it pretty much there…its a non confrontational way of acknowledging that you have seen someone or a greeting. “hello” was once a non specific questioning word (according to QI) for instance “hello what’s this then” or in a situation of surprise but became used as a greeting because of the telephone

so i guess its both greeting and question..which prompts a response….in a kind of I have seen you and i greet you have you seen me? at which point you either dive into an alley to hide or reply with a hi of your own

I’ve been to a lot of countries where they don’t say “hi”.

@eponymoushipster that’s true. In France they say Salut or Bon Jour etc… and in Sweden they just look at you (like your smell really bad). Like, duh

rofl @Cartman damn those swedes…true a lot of countries might not say hi but they will have a way of acknowledging people they know or meeting or whatever

@noodle_poodle in Finland, they kick you in the balls and expect you to take.

In Israel – it’s Peace when you are both coming and going. Shalom