You don't have the same soil type I do, that's obvious. Or the same tiller. My tiller has shoes on either side to set the digging depth. Maximum is about 3-4 inches. If I were foolish enough to take them off, then the side gearbox would give me maybe another inch before bottoming out.
I'm fine with that. Digging down an acre about four inches in one pass with a tiller would take 2-3 days on my soils. But I don't need my tiller to go that deep - that's why I still own my grandfather's 2-bottom plow and tractor.