It’s possible you leap into action every time you see the security notification, but many people don’t.
And, if you do, what’s the point of installing critical security updates manually? You often need to patch security problems as soon as possible to fix whatever the problem is.
After a patch is released, the security hole becomes public knowledge if it wasn’t already.
Attackers now know the problem and can rush to start exploiting it as soon as possible before people update.
Attackers know business and home users are often slow to update, and they can wreak some havoc in the time before people patch.
Just selecting “manually install updates” and installing updates when you remember to do so isn’t really good enough.
You want Windows Update to install those updates for you automatically.
For software like your web browser and browser plug-ins, you also want to leave automatic updates enabled — thankfully, that’s the default these days.