Ubuntu's (and by extension Canonical's) value is in its brand and existing customer relationships, neither of which Microsoft would have if they rolled their own distro.
I know there's a lot of MS hate around here, but that's ridiculous. Literally everyone would know what Microsoft Linux was within a month. It would be a headline on every tech site across the internet.
Besides, I just suggested one potentil buyer, I want to hear who else besides boring answers like Google.
Outside of the people who are going to hate MS no matter what, I don't know what reasonable CIO would have a negative connotation of MS Linux. They ALREADY maintain multiple branches/distros internally for things like their network OS; and anyone older than a millennial should know they wrote Xenix which was one of the most successful UNIX variants ever sold...