Because they don't actually sell their software, for the most part. They have deals with OEMs to preinstall Windows, and they get money for that. If they were to build and market their own hardware, those OEMs would be miffled to say the least, and their main channel for getting Windows out to the people would die to be or not be replaced, eventually, with the results of their own R&D and marketing and distribution. I don't believe Microsoft want to risk that gamble (yet).