That was a requirement twenty years ago when the framework wasn't bundled in the operating system and download speeds were much slower. It's really a non-issue now.
The whole point of .NET Core is to bundle the framework with the application.
I thought the whole point was to become cross platform. You can install the runtime on its own. Bundling an application and the framework isn't a requirement.
Some .NET Framework libraries that were ported to .NET Core are never going to stop being Windows only.
Being cross platform is an nice side effect of the whole rebooting process.
Or maybe there isn't a single "the whole point" and it can be useful for different reasons to different people?