I would have thought they would have had a commanding majority as well. Occasionally I'll buy something on GoG - but it's usually an oldy-but-goody for $5 or less. Almost every single game I have gotten through Humble Bundle has just been a Steam key.
I guess there's Origin, but I've only ever bought Mass Effect 3 from them.
Good luck even finding PC games in a box in a retail store...
EDIT: Derp, completely spaced Blizzard... that's where most of the missing money must be going.