There are plenty of things that are "in canon" that people write about - in fact, one could argue that fanfic is about writing what's in canon out of canon: is there any reason the canon pairing should be exempt?
Shows never show all the things that fans want to see - all the little details, whether gen, het, or slashy. Writing fanfic is about filling in gaps or elaborating on things that are there but aren't detailed, or aren't there but could be, or aren't there and could never be but are cool and hey, why don't we put them on a ship at sea and see how the characters interact there!
Re: Via Metafandom
Shows never show all the things that fans want to see - all the little details, whether gen, het, or slashy. Writing fanfic is about filling in gaps or elaborating on things that are there but aren't detailed, or aren't there but could be, or aren't there and could never be but are cool and hey, why don't we put them on a ship at sea and see how the characters interact there!