You didn't answer my question really. But at least you "sort of" admit that a PC is an "open platform."
Now why are mobile personal computers not "open platforms"?
Sorry, but no amount of weaseling answers will change reality on this one. Mobile devices are personal computers now and are the most important consumer product that anyone in society owns. You can destroy entire businesses by simply removing an app from either store. All of this power is in the hands of two of the largest corporations to ever exist and the business has no recourse to argue against these decisions. And they do happen.
Things change over time, as is this case with our mobile devices. What was once a luxury novelty is now practically required for day to day living. They have moved into a position of such wide reaching importance AND lack of choice that they absolutely need to be opened up, either by choice, or force of law if it comes to it. These are computing platforms and need to be on equal ground with Windows, MacOS, and Linux.