Individual Views are all struct values, that you can operate on in the usual ways through generic functions over the View protocol. One of the things about SwiftUI, though, is that it really prefers that you always return the exact same type of view. Early versions of SwiftUI did in fact have effectively a single View struct, and we found that this made the animation system in particular hard to use, because it was too easy to write things like:
condition ? Button().opacity().scale() : Button().scale().opacity()
which seem like they should work, but which are difficult to implement robustly in all cases, since the order of transformations and how they should be individually animated has to be reconstructed for all possible combinations. By basing the design instead on a protocol and generic wrapper structs, SwiftUI is intentionally trying to make the path of least resistance the one that gives the most robust results. The function builder magic is largely unnecessary, and beside the point to some degree; without it, you could still write if as a higher-order function, as @bzamayo noted , and we could construct compound views with regular constructors.