The problem is the operating system developers, instead of defining a standard component that could pull update lists from anywhere, just decided, "we shall make one store, and we shall be the gatekeepers"
Imagine if windows (or MacOS) had a control panel where you could paste "Update subscription URLs", and uses crypto to verify licenses (optionally), and implemented binary diffs and what not, and provided a standard operating system component for navigating, purchasing, and installing/updating software?
They were SO CLOSE with add/remove Programs, and it never went all the way.
IIRC there is the Install-Package Powershell commandlet that might almost be that? But, I think it's still locked to Microsoft's store.