Do you really want microsoft to release their code openly to the entire malware world?
No matter how security conscious or computer adept you are, consider for a second the fact that the majority of people aren't, and will be more than happy to unwittingly let their computer become a member of a botnet, spamming your inbox, and bogging down bandwith everywhere.
The reality is that for the majority of computer users in their ineptitude(not an insult, simply reality, most people dont know, dont want to know, and dont give a shit about anything more than surfing the internet, checking their email, and using word) it is not in the computing worlds best interest for their core operating system software to be open sourced.