Having the OS, the hardware, and the app store, along with a lot of the (almost mandatory) cloud services AND the most popular apps, all made by the same company.
All that choice is already there, just for a phone. Previously you'd choose a flip-phone and have it all locked together, including with the carrier.
When it really screams is if you log into iCloud then forget your password and it keeps asking you to input it. Like with every elderly member of my extended family.
- No, iCloud is not required to use an iPhone, this is fact.