The reason is that most drillers and all geologists know the facts about ground water: that it is almost everywhere. Non-scientists think water is in "underground streams" scattered here and there; that's false. When I was involved with drilling in limestone (the rock in the Texas story), the problem was not finding water; it was 1) finding clean water, because the large conduits are easily contaminated, and 2) avoiding creating sinkholes at the surface when the well is pumped.
People believe in water witching or divining rods, because people still believe in ghosts, psychics, and demons. If you go to more remote uncivilized areas, people can believe in evil spirits causing disease, drought, volcanic eruptions and bad luck. There are people that believe anything under the sun. Humans are a gullible bunch.