Tickle the ivories

I don’t understand why people say that a piano player is “tickling the ivories” when they play the piano.
I make pianos for a living, and I can report as fact that the white ivory keys are not ticklish.
Neither are the black ebony keys.
This is why you can dust a piano’s keys with a feather duster, and it won’t laugh.
In fact, none of the piano feels a damned thing.
The piano bench, on the other hand, is an oversensitive little prick.
That’s why we fill it with music books and sit on it to keep it shut.