We must not forget the legal side of this. Microsoft knows everything about intellectual property rights. If I were going to design a logo to be displayed literally billions of times across every screen running the windows OS I would make very sure it was beyond any conceivable infringement allegation. A generated smoke image might be up for some allegation of copying, that the generated smoke or some other aspect was "created" by the software. A physical photo shoot means zero possibility of any outsider having any contribution in the final image.