There are technical reasons, but as ever the real underlying causes are incentives. Companies realized that the OS is a profit center, something they can use to influence user behavior to their benefit. Before the goal was to be a hardware company and offer the best hardware possible for cost. Now the goal is to own as large a slice of your life as possible. It's more of a social shift than a technological one. So why would a company, in this new environment, invest resources in making their hardware compatible with competing software environments? They'd be undercutting themselves.
That's not to say that attempts to build interoperability don't exist, just that they happen due to what are essentially activist efforts, the human factor, acting in spite of and against market forces. That doesn't tend to win out, except (rarely) in the political realm.
i.e. if you want interoperable mobile hardware you need a law, the market's not going to save you one this one.
Because that's what customers want to buy. People are paying premium iPhone prices for hardware with mediocre specs and then the hardware sells out when someone like Purism or Fairphone actually makes an open one. How many sales would you get if you did the same thing on a phone that was actually price/performance competitive with the closed ones?
Meanwhile all of that "profit center" talk is MBA hopium. Nobody is actually using the Xiaomi App Store, least of all the people who would put a different OS on their phone.
The real problem here is Google. Hardware attestation needs to be an antitrust violation the same as Microsoft intentionally breaking software when you tried to run it on a competing version of DOS and for exactly the same reason.
Any specific individual that is high value will walk into a store and buy from stock.