It's quite different, because
swift build only builds standalone command-line executables for macOS. iOS (as well as macOS, for non-command-line apps) requires handling assets/resources (invoking tools like
ibtool), compiling localized strings files, configuring Info.plist files, and then copying them into the appropriate
.app bundle structure, in addition to signing that you already metioned. Then, if they wanted to support other targets than just basic iOS apps (like app extensions, or framework bundles), then they'd have to handle the differences between those kinds of bundles as well.
Could all of those features be added to SwiftPM? Technically, sure. But it's probably out of scope for the Swift project, since none of those features above are strictly about Swift, and it would require the SPM team to constantly chase new OS product types and duplicate work already done in Xcode. If you want to see a bit of how complex reinventing this can be, a good place to look would be the Bazel rules for bundling Apple apps.
It wouldn't surprise me at some point to see some kind of support for included assets/runtime data files as part of a Swift package, but that's a lot less complexity than building full apps.