Board Thread:News and Announcements/@comment-4224349-20141031210600/@comment-4536113-20141102152243

None of the other crystal gems eat besides Amethyst (they can, but it's obvious that they don't as seen in Beach Party), so that could be a reason that they fuse because Amethyst would obviously not pretend to be Steven's mom very well (for reasons stated above, lol).