It's funny because Notch said it can happen, but Jeb said it can't.
Perhaps it was removed?
Like with other features he said he will or probably will but ended up not adding it. The tweet where he says that he is going to make them have a small chance to spawn in the overworld was before the update.
they might be able too because when i was playng there was a zombie pigman standing at the portal in my base, i heard his grunting but i didnt think it was really one until i went to my portal, it might be a glitch