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.