because you will not update until forced in most situations.
you simply are not aware of every security fix or vulnerability found, and therefore can't know when you should update.
in the early 2000s there a few very large profile windows viruses that spread like crazy, despite patches being released months or years earlier. the viruses took out networks across the globe and caused considerable mayhem for days.
congress had hearings and several tech companies testified, including Microsoft. congress wanted to know why users weren't forced to update their computers, and Microsoft said that they can't control what their users do.
congress blamed Microsoft for users not patching their systems.
so this happened a few more times and Windows Update was improved continuously going forward.
now you're forced to update, because people who thought they knew all about security failed to update when they should have.
now, users blame Microsoft for forcing them to patch their systems.