The tone of your question is very argumentative, by my reading. It's not even clear what you're asking because your tone comes across as disingenuous.
To answer the question I think you asked:
The reason MS produced their own free AV product are pretty simple: Increasing security makes Windows more attractive as an OS.
Publishing your own AV solution means your customers are not required to buy third-party software on top of what they already paid when getting the OS. When AV companies have all this marketing saying how viruses and malware are so prevalent but MS doesn't offer any real protection, it looks extremely bad.
Simply put, MS's customers shouldn't have to pay a third party extra money just so the OS is secure when they're already paying ridiculous amounts of money for the OS itself. It makes MS look like they don't care about their customers. Whether they actually care or not is irrelevant, but looking like you don't care is absolutely fatal. MS might be a monopoly, but that doesn't mean they don't need to worry about competition from Apple or Ubuntu.
The horribly shoddy programming that most AV programs suffer from which cause tremendous performance issues also makes Windows look bad. What good is it to get the shiny new computer with the shiny new OS if it's going to run just as slow because of the same shitty AV program? Remember when Norton was good, and now it's shitty bloatware? Remember when AVG was good, and now it's shitty bloatware? I suspect we'll be adding more programs to this list like Avast! and Avira eventually.