One of the reasons users will have to wait so long for a 120Hz display on the non-Pro iPhones is probably because Apple wants to maintain significant differentiation between the latter and the more premium iPhone models. Only when the Pro models adopt under-display Face ID will the non-Pro ones receive the 120Hz treatment.
A 60Hz display on an $800+ smartphone was a tough sell in 2022, and it will be interesting to see Apple try to pull it off again in 2023 and 2024. Especially if the company hikes the starting prices of the iPhone 15 lineup as most leakers expect. Then again, Apple seems to be able to get away with just about anything.