Owning your workflows
Sure, by doing that, Apple has stronger control over the ecosystem, and therefore, over the developers' apps. However, there are situations when the proposed method doesn't work for your scale. Then you need to come up with creative ways to accommodate Apple's processes to your needs. An excellent example of that is replacing Xcode's build system with Bazel and figuring out how to get Xcode to compile with Bazel instead of using its build system. The result of that setup looks more than a hack. It's brittle. One day it works; the day after, it doesn't. And because Apple is so distant from many real challenges developers face, they continue building around their utopian vision of app development. That one where apps are small, a few targets, and dependencies are seamlessly distributed via Swift Package Manager.
The reality is far from that. The day-to-day is way more convoluted than what Apple thinks. And because Apple doesn't embrace that and provides more flexibility to customize processes, teams have no choice other than to wait for the next WWDC to see if Apple decides to tackle the issues they are facing.