Perhaps because humans designed them that way.
“The direct or indirect human artificial selection process made the dog bark as we know,” said Csaba Molnar, formerly an ethologist at Hungary’s Eotvos Lorand University.
Molnar’s work was inspired by a simple but intriguing fact: Barking is common in domesticated dogs, but infrequent if not downright absent in their wild counterparts. Wild dogs yip and squeal and whine, but rarely produce the repetitive acoustic percussion that is barking. Many people had made that observation, but Molnar and his colleagues were the first to rigorously investigate it.
Humans Guided Evolution of Dog Barks | Wired Science | Wired.com/continue reading