If Jerry and Elaine started going out, the show would need to be much more "serial" than "referential". Currently the gang makes references to past episodes, and even to their own popularity ("I'm Kramer!"), but given the constraint of having to remember past epsiodes for the developments in Jerry and Elaine's relationship might take away from the creativity of the show. The rule of Seinfeld seems to be that the characters never learn. This isn't the Bradys, there are no morals that aren't forgotten by next Thursday night.
Besides, if Jerry and Elaine were an item, we'd never meet the string of funny people they DO go out with!
Sean
(Love the Drake!)