The reason is exactly why MS has improved their security over the years. One of the things they've done is made automatic updating a mandatory feature of the OS

The reason is exactly why MS has improved their security over the years. One of the things they've done is made automatic updating a mandatory feature of the OS. People can't just lazily turn updates off anymore because they can't be hassled for a 45 second break for their computer to maintain itself. Were these people running Linux, a lot of them would be doing the same thing, with the same results. Windows has a lot of botnetted computers because Windows runs the vast majority of computer systems out there. The neverending security updates is part of the difficult balance MS has to take between compatibility and security. Fixing a security problem that breaks a buggy program written 20 years ago by a company that no longer exists suddenly becomes a support issue, because there are a lot of people who don't want to hear that they have to upgrade their copy of PrintShop. MS releases security updates in part because they audit their code, and are making strides to get rid of a lot of the cruft. Windows 10 pulled a lot more services out of kernel space and into user space, for example. They're doing so while being conscientious of user needs, instead of telling the user to just code the fix for older programs themselves. In your small OS, who do you go to for support if something breaks? Who will you go to for support when a program from today breaks ten years from now? These are responsibilities many open source programmers will slough onto the end user, while they're working on the Latest and Greatest PulseConsoleSystemAudioKitD.