what is the reaon mr. Arable says that they cant hear the animals talk

This is actually from Chapter Eight.

"Maybe they do talk," he said. "I've sometimes wondered. At any rate, don't worry about Fern - she's just got a lively imagination. Kids think they hear all sorts of things."

"Mr. Arable grinned. "Maybe our ears aren't as sharp as Fern's," he said."