Why, according to “Tom,” did Wilson’s plan not work out?

According to Tom, Wilson's plan didn't work because the knife in question doesn't exist.

I think so," said Tom. "There can't be any doubt of it. Have you ever seen that knife?"

"No."

"Has any friend of yours?"

"Not that I know of."

"Well, I begin to think I understand why your scheme failed."

"What do you mean, Tom? What are you driving at?" asked Wilson, with a dawning sense of discomfort.

"Why, that there isn't any such knife."