A seatbelt does not lose its ability with time (I have never heard about "changing the seatbelts"). So they don't need updates. However, software that connects to the internet "loses" security with time (as time passes, more vulnerabilities are found). That's why we need updates: to maintain our non-zero security level. Then you could argue that we could have smaller updates with fewer patches, because what we have right now is overkill. And maybe some of those updates don't really matter much, indeed. But some do.

And for those, you need security updates.