And it has to sync when your devices might never actually be turned on at the same time. And you want it to sync when you're traveling, and you don't want to open your home firewall for security reasons.
So it requires a single, centralized, always-on server as the source of truth. And given the speed of light, whether that server is 3 miles or 3,000 miles away is largely irrelevant.
And writing additional code to sync directly when devices are on the same LAN and turned on at the same time, as a special case, is not only entirely redundant -- but you still have to sync with the server, since you generally want the information backed up as well in case you lose your devices, or they're stolen, or damaged. Using your examples of Contacts and Calendar, those are often some of the most important things in someone's life to keep synced remotely.
Does that answer your question?
I would mind that since its local and I prefer to not be dependant on external hosting services wherever I can have a more accountable and static solution a little closer to home
I, and everyone I know, wants something that always works, all the time, no matter where they are, that they never have to think about.
Not something that they "have to share in ensuring", not something they need to "set an automation". For almost everyone, external hosting services are a feature rather than a dependency, because homes are subject to fires and natural disasters. And nobody wants to deal with finding a friend across town who will host your backup NAS that you'll set up and periodically maintain.
There's just no market for it from any major consumer company.
And for most people, it works fine, but when you hit edge cases you're dead in the water.