Because Apple doesn't let you build static libraries that contain Swift. Unlike Objective-C, Apple doesn't ship the Swift standard runtime libraries with iOS. This decouples the language version from the platform version. When you build an app with Swift, you're responsible yourself to ship them. By default, Xcode uses swift-stdlib-tool to handle copying the Swift runtime dylibs, but the tooling falls short when attempting to ship frameworks that use Swift with an app that is Objective-C only. Your app executable and the frameworks you ship will all use the same set of dylibs, which are embedded into the Frameworks subdirectory of the application bundle.
First, that's because you can't link against different versions of the standard libraries. Furthermore it is desirable to embed them only once and not multiple times, because of constraints to memory size and network speed, which are relevant for distribution. You can make CocoaPods integrate to your project via frameworks instead of static libraries by specifying use_frameworks!. If that's not present, you won't be able to integrate dependencies, if you depend on a pod which includes Swift source code.
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