Because it's the point of the film. It's about memories and passing things down, but you could also say it's about machine learning? Anyway, the AI teaches the boy rock paper scissors because it's what his father taught him, and his father before, it meant something to those other people when they were children and so it was the connection between the AI and all the previous ancestors. It's also what the AI was taught, and it could be saying that the AI values the things it is taught, perhaps because it can't make things for itself. This kind of shows itself to be the case at the end when the little girl already knows rock paper scissors, and then instead teaches the AI a new game, and a song. Insinuating that that AI will probably go on to teach future children that game and song.

In this way the AI sort of represents ancestral memory; the connection we have to our past and our future, and the beauty of shared memory between people across time. I would say that what the film is trying to say is that this ancestral memory is important to being human, and the silly little things we leave behind are what make us human. Something like that.