We all know the basic plot of the show so I'll skip reiterating it.
The manga series has been running for about 20 years, and Steven Den Beste expressed some surprise that Keiichi and Belldandy hadn't done much more than kiss and hold hands over such a long time...and that got me to thinking.
Keiichi's wish is approximately this: he points at Belldandy and says, "I want someone like you to be my girlfriend forever!"
...subject to the vagaries of translation, of course. But a Japanese person won't necessarily say "you" even when they mean you; they might say "someone like you" or "someone like [your name]" or any of a thousand other variations. For Keiichi to tell Belldandy, whom he just met, "I want you to be my girlfriend..." might be a bit much, even for a joke.
The wish is entered into Yggdrasil and approved. Then Belldandy rushes to the phone and frantically calls God.
...because Belldandy has been cloned; the clone finds herself in a pocket universe with Keiichi and duplicates of the others in his life.
We don't know what God says to Belldandy; it's possible He says, "You are the clone and you were created specifically to make his wish come true."
From that point on, it's the Ultimate Force at work. It's not just that the series has run so long; it's a plot point: she is really going to be his girlfriend and it's going to be forever.
Keiichi is in a pocket universe where time is essentially standing still. There is some progress but each year takes a handful of years to pass; the exact ratio is unimportant as long as we bear in mind that Keiichi's freshman year of college could have taken five or ten years (or more) for him to complete--unbeknownst to him.
Part of the tone of O!MG! is timelessness; the series is highly episodic in nature, with each crisis standing alone as a story even though prior stories may have introduced minor changes into the canon. The characters hardly change at all.
The crises serve two purposes: they keep Keiichi from getting "too close" to Belldandy; the interference of Skuld and Urd's failed attempts to help serve the same purpose. They ensure that the relationship never goes beyond "boyfriend-girlfriend" because Keiichi wished to have a girlfriend forever. The crises also keep Keiichi from noticing the glacial pace at which things progress.
Everything is happening to ensure that his wish is fulfilled--all of it, 100%. Belldandy (or, rather, her clone--"someone like" Belldandy) will be his girlfriend, forever, the only way such a thing could be: in a pocket universe where nothing ever really changes.
So what happens if the contract is terminated, I wonder? Will Keiichi find himself back in his dorm room as if nothing had happened? Or will he find himself decades in the future, still 20-something?
I also wonder about the maturation rate of goddesses. How old is Belldandy? How long ago was her childhood? How long does it actually take Skuld to grow up? Skuld might be aging in real time and Keiichi simply hasn't noticed that it's been 20-odd years since she arrived at the temple. Hasn't, because the Ultimate Force is acting to ensure he doesn't notice, because if he does the jig is up and the entire house of cards comes down--and then his dutifully-approved wish has failed to come true (and God alone knows what the consequences of that may be...).
It's probable that this is a lot of stew from one oyster. O!MG! is a popular series and Fujishima has no incentive to end it; it's a fan favorite, anyway, because Belldandy is one of the ultimate otaku dream girls. That alone is enough of a reason for him to keep the "status quo" going--but I like to think about the "in-story" justification for things, and this seems a reasonable extrapolation to me.