My guess is it’s been bred in as an unwelcome side. Labs are retrievers and I can tell you mine is like the little kid who is not happy unless he is putting something in his mouth.
It’s really sort of funny. His mouth is never empty - it may ba a sock, a water bottle, a paper plate. When I let him out in the am, he turns back from the door and runs around the house searching, until he can find something to carry out. I didn’t have to teach him to fetch. He knew from the time he could run.
I suppose that’s as good an explanation as any. But fer cryin’ out loud, look at post #14. That one has a habit of swallowing ROCKS! As you noted, little kids instinctively like to put things in their mouths too, but they don’t *swallow* things like rocks, nylon leashes, and all sorts of other random objects that have no hope of passing through their digestive systems. They often accidentally swallow very small objects they’ve put in their mouths, like coins, buttons, tiny pebbles, etc, but that’s very different.
I haven’t had a dog since I was a little kid — I’ve been a cat person, largely because they’re easier to take care of. I do plan on getting a dog somewhere down the road, but I’ve ruled out labs on account of the swallowing-stuff problem. It’s funny until the dog is writhing in pain and the vet informs you that it’s going to cost several thousand dollars to save the critter.