Although raven gives a somewhat reasonable explanation :

"They were high school sweethearts, but things change. People marry their high school sweethearts and then they grow up and things change and they get divorced and things are good because there's still friendship there," Raven explained.

i think a lot of us who grew up on the show still want them together, it would make a great heart warming episode,they could even play blue Rain In The Scene .. the show is already great,but i feel like this is a big problem..