What you’ve linked is CoreFoundation’s retain, Objective-C’s can be found in https://opensource.apple.com/source/objc4/objc4-787.1/runtim... (look for objc_object::rootRetain).

The short answer for why it can’t just be an increment is because the reference count is stored in a subset of the bits of the isa pointer, and when the reference count grows too large it has to overflow into a separate sidetable. So it does separate load and CAS operations in order to implement this overflow behavior.