If you build yours, say, twenty-four inches on top of the ground, that is two feet of root space for your food. And, if you have a screen at the bottom to keep out the gophers, moles from the roots, anything beyond two feet of root growth stands to be eaten by the varmints anyway. This is my take on it. Otherwise, we can plow the earth long and deep, install thick screens all along the trough to keep out the pests and do away with the idea of raised beds.
Of course, below each raised bed, one could dig down deeply, install the pest screen and build it up from there.