Why? Because that bark has been bred into them through rather ruthless selection. Working terriers “go to ground”* meaning they pursue their prey into their burrows. These burrows can be long, twisting and snug. Once in, there may be no way for the dog to get out. Then they must be dug out. By hand. By the shovel-toting “terrierman” (or woman).
Here is a Jack Russell terrier demonstrating, in 34-seconds, how that can happen.
Getting stuck is problem enough. But there is another. I tagged along on a rabbit hunt with a small band of terriers. The dogs raced around, into brush and tall grass, questing tirelessly. Suddenly one dog was gone. Soon after we heard muffled barking. Following the sound, we found the hole the dog disappeared down in a thicket of briars. Lying on her side in the brambles, the human was able to reach her arm far down into the hole, grab hold of her dog and drag him out rear first.
If her little dog had not barked, we probably would have never found him. And if that little dog was truly stuck down there, he would have died struck down there, as many terriers have through the centuries.
So “Silent Sam” left the gene pool quickly (and rather grimly) and “Barking Bob” stayed. And, as I like to joke, many working breed terriers only bark when they are excited, frustrated, happy, eager, lonely, etc.
Oh, this doesn’t mean you can’t teach your dog to quiet down quickly, it just means you’ll probably get lots of chances to practice!
Now you know.
* Here are the AKC terrier breeds that can compete for Earthdog Titles: Australian Terrier, Bedlington Terrier, Border Terrier, Cairn Terrier, Cesky Terrier, Dachshund, Dandie Dinmont Terrier, Glen of Imaal Terrier, Parson Russell Terrier, Lakeland Terrier, Manchester Terrier, Miniature Bull Terrier, Miniature Schnauzer, Norfolk Terrier, Norwich Terrier, Rat Terrier, Russell Terrier, Scottish Terrier, Sealyham Terrier, Silky Terrier, Skye Terrier, Smooth Fox Terrier, Welsh Terrier, West Highland White Terrier, Wire Fox Terrier, Yorkshire Terrier