The required measurement is of resistance; why can’t I use a multimeter?

For the same reasons that a continuity range on an insulation tester should not be used. Measurements made with a DC multimeter are subject to distortion by electrical noise in the soil.

With a multimeter, one can measure the resistance of the soil between a ground electrode and some reference point, such as the water pipe system, but a fault current may encounter a higher resistance.

Fluke FLUKE-115 Electrician True-RMS Digital Multimeter