Again, because this makes declarative UI cleaner to write, or if you prefer to put it this way: reduces clutter.

I could create button templates then make slightly different copies of those templates without having to subclass Button or attaching .copy()

SwiftUI is all about succinctness.

Why is it all about succinctness? Swift has never been about succinctness - it’s always been about clarity, without over-verbosity. Sometimes, it just takes a decent number of characters to describe something. That’s not necessarily a flaw, especially if it makes something easier to read.

Seeing .copy() makes it much clearer to understand that what I’m receiving is a copy of something.

If you want a Button template, either extend Button with a method taking the parameters you want, or write a curried free function or something. callAsFunction adds absolutely nothing to the language, bar confusion at the call site.

Playing code golf with the language syntax isn't the point, clarity and expressiveness is.