They’re not the forever-open platform you’re purporting them to be.
Look at the increasing push for device attestation via SafetyNet or whatever it is called now.
That’s why even Adam Smith himself said that it only works in well-defined markets! It’s the job of governments to define these, and phones are so essential to our lives that they should be thought of as roads, instead of as an average private company.
Imagine saying that about everything else in your life. Don't want to buy a Ford car? You can buy a Toyota. There are no other car brands in the entire world. IHG, Hyatt, Choice Hotels, Best Western, Wyndham, Radisson, they don't exit.
Don't want to stay at the Hilton? You're in luck, you can go to the Mariott, that's it.
Don't want to buy a t-shirt at Target? Well, Walmart sells t-shirts, and nobody else sells any t-shirts.
This would be astoundingly ridiculous anywhere else but technoloy. Even credit card processing networks have four options (Visa, MasterCard, Discover, and American Express).
As an example of how this works in other industries, US antitrust regulators automatically decline any merger/acquisition US airlines that would put their marketshare above 20%. And that's after deregulation!
IMO any market that has less than ~3-5 viable purchase options for customers needs to have extensive government intervention to keep the market fair and innovative.