Why not? You saw Macy and Gimbel shaking hands. That wasn't possible either, but it happened.
6 seconds sound clip from the Miracle on 34th Street (1947) movie soundboard.
You can hear this line at 01:10:55 in the Blu-ray version of the movie.
Quote context
[...]
- Read all about it! Gailey throws bombshell in New York Supreme Court.
- Oh, I didn't see this... Front page. Good, good.
- You're not serious about this.
- But you can't possibly prove that he's Santa Claus.
- Why not? You saw Macy and Gimbel shaking hands. That wasn't possible either, but it happened.
- It's the best defense I can use - completely logical and completely unexpected.
- What about your bosses, Haislip and Mackenzie and the rest of them? What do they say?
- See?
[...]