Did you know that farmers in the American south still use divining rods to find water?
Well, they may or may not, actually, but one of my relatives is of the impression that they do, and often brings this up as evidence that water divining (AKA water dowsing AKA water witching) works. To be fair, she would know if they did at least up through the 1950s, so we can take it as given that at least some farmers in Arkansas were using divining up to the middle of the last century.
Don't misunderstand me, I am not picking on my relative. This is a common lapse in critical thinking, committed by most of us at some point in time, the "but group X wouldn't do activity Y if it didn't work!" argument. It seems persuasive - after all, people who are reliant on something (as farmers are reliant on water) wouldn't want to waste time and energy on a non-productive effort to get the resource.
But people do. People use all manner of things that do not work to achieve various ends. Doctors, who should know better, hyper-dose vitamin C when sick despite the fact that the medical data shows that this doesn't really help; athletes wear holographic bracelets to improve performance, despite the fact that they are nothing but an "applied kineesiology" prop and don't actually work (by the way, you can get a nifty placebo bracelet for far less); and huge swaths of people would rather pray to a myriad of non-existent entities for safety than to do such basic things as wear seatbelts.
All of it is a waste of time, energy, and resources. And yet people do it. Well-trained, smart, and cautious people do.
Our 1950s farmers who rely on water divining are a good example of this sort of thing because of the fact that the farmer is the last person that you would expect to be wasting time and energy on needless superstition. While many of my urban friends have an image of the farmer as a goofy yokel who lacks in knowledge, nothing could be farther from the truth. Farmers tend to be very intelligent and practical, they are generally good at using resources wisely, they are mentally flexible, they usually are good with mathematics, are knowledgeable about practical applications of mechanical engineering, have a solid knowledge of field biology, they have always been inventive, and from the mid-20th century onward they tend to be very well-educated. They have to be, farming is a tough profession that will devour anyone who lacks both the general grit and the intelligence necessary to carry it out.
So why were these guys using water diviners, a profession that has been shown time and again to be a sham?
Well, there's alot of reasons why they might. The first may be that the diviners do find water - but not because of any special magical powers that they claim to have. One of the things that I frequently have to do is predict where archaeological sites will be, and a common thread amongst residential sites (campsites, villages, etc.) is that they are close to water. So, I have learned to look at a landscape, whether on the ground or with maps, and figure out where the water is likely to be most plentiful and/or accessible. I am not always right, but I am often enough. So, if I can do this in areas where I have only a passing knowledge, it's no stretch of the imagination that someone who is very familiar with an area will be more accurate more often, no mystic abilities needed. Of course, human psychology being what it is, these people may be convinced that they know where the water is for magical reasons and not because they simply have gathered basic information. Also, in most regions with water, if you dig deep enough, you'll hit it, so the water diviner can't help but find water...even if it isn't very convenient water to get to and you'd be just as well off letting a 6-year-old throw lawn darts to figure out where to dig your well. But, hey, regardless of self-delusion or efficiency, they find water and that's all that is necessary.
Another possibility is that it may simply be tradition to use a water-diviner. People in tough environments often rely on traditions and rituals to help them make it through the day - these need not be practical, they simply need to make the person feel like they have more control over their environment than they really do. While farmers often work in non-marginal environments, the resource strains that they feel are often similar to be people in less ideal places. By relying on water-diviners, they may be able to put worries about water aside and focus on other matters, allowing them to get things done - so what seems impractical may, in fact, be very practical when considered in its broader context.
There's also the confirmation bias, something at work in many decisions that all of us make. We forget the misses and remember the hits (or the other way around), making us believe that a view that we favor is the correct one. So, if you remember the time that Joe Smith found water on land that was thought barren, you may forget the 20 other times that he couldn't find water on land where it was clearly readily available.
And, of course, there's good ol' social pressure. It may be that paying the local diviner is a good way to keep money in the community, so you do it for that reason. It may be that everyone else believes in water divining, so even if you have your doubts about hiring someone to wave sticks about, you do it anyway in order to keep in line with the existing social order.
Now, if this particular gaffe in thinking were limited to water divining, I'd probably just ignore it. But it isn't. I note several other examples above, and if I were to sit down and look for more, it would be easy. It's common for us to assume that some phenomenon is real simply because people who we assume should know believe in it. However, as is shown by water divining, even the belief of professionals doesn't make something real.