It has always amazed me how popular this programme is. Even when I was a child, it bored me to tears and I have never been able to watch a full episode.
I don't know how adults can watch this programme.
So to say it is "the greatest programme ever made" is only your opinion.
Humor is hugely subjective and there's not much chance of convincing someone that something is funny if it's simply not their preferred brand of comedy. To each their own.
But as for adults enjoying it, the show was written in its best years by a bunch of atheist Ivy League types. Their humor included both characterization-dependent stuff, the utterly absurd (much of this thanks to the animation medium) and a lot of social commentary that is massively disdainful of middle America. It's this mastery of multiple comedy forms that kept the show fresh for me (an adult for many of the years I've enjoyed it) for so long.
They eventually lost good characterization and intelligent social commentary; all that remained was a 'zaniness' that was a shadow of what they'd previously been capable of. They also seamlessly integrated a ton of cultural references that geeks would appreciate at their peak; now these references frequently are
the story and they're also obvious references these days rather than the classic and/or obscure stuff they previously used. Needless to say, the show has been shit for years now and I myself haven't watched a full new episode in a very long time.