The reskins are most likely being done because Apple is slowly switching different views in macOS to be built with SwiftUI (rather than UIKit), giving Apple an additional reason to also update the UI at the same time.
Rather technical, but underlying it's helping Apple to move SwiftUI forward by eating its own dog food via macOS updates.
For those unfamiliar with the term "eat your own dog food"...
Eating your own dog food is a phrase that refers to the internal use of a company's own products or services in its day-to-day operations. The idea is that if the product is good enough for consumers, it is good enough for its employees to use on the job.