It goes without saying that they are optimizing for engagement with their platform/app and user growth, just like every last digital huckster on the internet.

To keep people hooked while making them feel that the app is working, even though they are not getting their end result.

You have it backwards. The apps aren't training the human preferences, the human preferences are training the apps. There have been a LOT of different dating app ideas, and all have been competed away except for the ones that provide users the biggest perception of optionality.