The reason they made it a wanking gesture is in case you’re texting your ex while wanking with your phone in your hand.

The original idea was to undo all the texts you’d already sent. Unfortunately, due to unforeseen constraints on the nature of spacetime, Apple was unable to deliver on their text-message-undo promises, and went for typing-undo instead.

Then they never changed the gesture, and it’s still wankery to this day! Just a little bit of tech history for you there.