The digging is the problem, not the foundation
At the roadside, digging is the costly and risky part: it opens up the verge, requires signage and lane restrictions, and means running into whatever runs underneath. Then comes pouring, waiting for the concrete to cure and finally reinstating.
With screw installation there is no digging. The machine drives the screw at the planned point and the ground around it stays closed: no spoil to take away, no reinstatement to do afterwards.