We hear about half elves all the time but shouldn’t half dwarves and half gnomes be more commmon?
That terrifies me…
I don’t believe there are any half dwarves or half gnomes in canon. We’re not even sure if those CAN exist, although theres really no reason they shouldn’t be able to.
We really don’t. They’re actually rare.
Half breeds in general are.
Why would they be more common? 
What if dwarves are half gnome, half human?
Because they all share a lineage as titan construct. Compared to elves who come from trolls.
Half-breeds are already rare and most races cannot produce half breeds.
There’s that Gnome/Human and Gnome/Goblin couples that have been trying but neither can have children together. I feel for them since Elves/Human and Draenei/Orcs and Orcs/Ogres can create offspring.
I mean, Moira’s son is half bronzebeard dwarf, half dark iron dwarf.
They aren’t technically the same species.