Why doesn't MS just spend the time building an OS that is not as vulnerable to viruses and malware?
Since they won't do that, why not just build the AV stuff in at a lower lever so that it is not obvious to the user that they are even running AV? MS likes to sneak stuff in under your nose anyway, why not something that will make the OS safer and more stable?
Oh yeah, it's probably because of some conspiricy like they are the major shareholder in Symantec or Norton...