My theory:
They wanted us to think, "Why did we follow these people for all these years?".
I thought the final was simply painful to watch; it was pretty sad that I wanted to change the channel for most of it. Why not go out with a bang, and people will say, "Boy, that was a really good show while it lasted, but it was time to go."
Now I'm leaving with a sour taste in my mouth.
http://members.aol.com/sonysdds1/seinfeld.html talk with others about the final.