> Couldn't the extension do that itself? Why does it need to be a browser feature?

The reason for wanting such notifications in the first place is because new owners often do dodgy things, and a company that specialises in buying up popular extensions and cramming them full of spyware typically doesn't want you to know about the change of ownership.

So extension authors can't be relied upon to provide this feature. The ones you particularly want to know about go out of their way to hide their involvement.