Unilateral bargaining power. Facebook is willing to write an app for iOS that doesn't track people as well as they'd like, because Apple is there on the other side of the table, with 40% market share, saying "do it our way or don't do it at all."
Compare/contrast Windows PCs, where everything really is ultimately "whatever the user wants." You can arbitrarily modify the executables that run on your computer, sure. But can you get a modded game to let you play online, without running some Intel SGX rootkit that verifies the game's code signature and watches every HID input you make to ensure you're not cheating? No, you can't — because despite owning the PC itself that the software is running on, you don't have a negotiating position that enables you to tell the company that runs the servers that that game connects to for online play, to allow a non-rootkit-monitored version of their game client to connect to them.