Linguistics: Why do interjections differ?

Because, contrary to what you might think, interjections are not always pure spontaneous exclamations from deep in the neural cortex, that are universal to all humans.

A few are; as I noted in Nick Nicholas’ answer to Are there any short expletives that sound the same in different languages?

Nick Enfield [Page on sydney.edu.au] (who I did linguistics with, and boy does he look different twenty years on) just got an Ig Noble [Improbable Research] for claiming the universality of Huh? (The Syllable Everyone Recognizes, Is ‘Huh?’ a universal word?)

Of course the realisation of Huh? does differ by language; in the Mediterranean, for example, it is E? But the general idea is a mid vowel (as close to a schwa as your language allows), with a questioning tone.

However plenty of them are culture specific; they may not be arbitrary in themselves, but the choice of which interjection to use can be; and in fact interjections can be borrowed between languages, just like any other word.

Two instances from Modern Greek.

• “Ouch” in Greek is traditionally [ax, ox]. English [autʃ ~ auts] has now been borrowed into young people’s Greek, from TV.
• The Greek sneeze interjection is [apsu]. I’ve just discovered that the Turkish interjection is [hapʃuː], and [apsu] is just [hapʃuː] nativised to Greek phonology. (How is the sound of a sneeze written out in different languages?)