> ditch diggers in excess of supply, digging ditches would pay very well

Only if ditches are valuable enough.

Otherwise everyone does without ditches (long drop, sump hole), or find solutions that don’t need ditches (eg wireless or overground cables), or invent new ways of making ditches that are not digging (ditch witch, suction vacs), or other smart workarounds (trenchless water pipe, fibre optic horizontal drilling).