Because in the long run it's more convenient to enter into a relationship with Microsoft if you hope to use its products in an ongoing way. And that serial number is not a thing. It's a contract. It is the symbol of a relationship. Interactivity is also central to the future of creation.
Performance is a form of interaction.
The reason Deadheads went to concerts instead of just listening to free tapes was that they wanted to interact with the band in meatspace. The more people knew what the concerts sounded like, the more they wanted to be there. I enjoy a similar benefit in my current incarnation. I'm paid reasonably well to write, despite the fact that I put most of my work on the Net before it can be published. But I'm paid a lot more to speak, and still more to consult, since my real value lies in something that can't be stolen from me my point of view.