Earth boxes and root space

Can someone explain to me how earth boxes supposidly do so well if they have so little root space for the crops to grow in, this seems to me like it would stress the plant resulting in more diseases?

You mean like a raised bed?

My beds are 30cm deep with an open base directly onto the ground.  Therefore the soil is 30cm deeper than "ground level" Of course this means that the plants have an extra 30cm of root space.

yes the product, i see alot of picture of corn plants with 4-5inches of space and im wondering how its even possible to plant them so close without them trying stressing over root space???