Why does no Half-Dwarf Race exist?

We have Half-Elves, Half-Orcs, and (I hope I remember it right) in the Kaer Maga Handbook they even mention that mixing halflings and humans often ends up in handicapped children. But I have never heard of Half-Dwarves in Golarion. Is there a reason why they do not exist in Golarion? Or do they exist and I have simply not found the source yet?