Venturing into a product category just because your competitor is doing the same is not great motivation, and is a distraction at best. Microsoft has no interest in selling computers[1] in large volumes: it's long-term OEM partners may not appreciate Microsoft cranking out computers in anger. The Surface line seems to be a Halo product for Windows, or examplar to OEMs (and I'm guessing Microsoft mostly works as an integrator of existing parts made by other manufacturers).

1. Renting them out via Azure definitely interests them.