There are some. You just have to poke around.
This is how I found them on my iPad: From the store I hit the "more" button (top left). This brings up various quick links in the store. Select "Free comics." From here just scroll down and you'll see a Free Comic Book Day category. There are currently 14 books to download.
I recommend Molly Danger by Jamal Igle. Top notch all-ages super-heroing featuring a young female protagonist.
Edited to add: Just noticed you said from the "big" publishers. IIRC last year the DC FCBD book was added a week or so later. So I would guess it's just a matter of waiting.