I guess it's a goofy (and funny) depiction of a typical American family and people surrounding them. Like the other comments say, it's just a comedy and doesn't always have to make sense.. as long as it makes people laugh. Hope this answers your question!!
What does being from Australia, in context with why you don't understand it, matter? I'm from NZ and I understand it perfectly fine. It's a comedy show... it's supposed to be funny.
I think the Simpsons are not only funny, they often have subliminal messages in the episodes. After watching some of them twice or three times, I noticed some social and political criticism. Much of that can probably be only understood by US Americans and of course there are some episodes that are just for fun.
