It’s a good question — I have no idea. If I had to guess, I’d imagine it’s largely about tradition. If Apple stopped shipping them with new products, I could envision a revolt similar to if Google took away the “I’m Feeling Lucky” button (in that, while not that many people may use these things, they’re expected to be there).
I can’t even begin to imagine how many of these stickers I’ve pilled up over the years. Dozens? Hundreds? And I think the last time I actually put one of them anywhere was the very first one I got (with an iPod way back when).
In a way, they’re viral real-world marketing. I still see them on the bumpers of cars quite frequently.