There are so many factors. I think the biggest one is that the developed world looked at women and said "hey, they are just as smart and capable as men and if they work at companies we have 2x the workers" which is obviously true but what it leads to is a DINK society - and it locks you in. It is just much, much harder to raise children when both parents work and they don't live near their parents and other familial support. Add that into your observation that the world is more fun and selfish and it multiplies.

A lot of the decrease is also correlated with access to birth control which drastically reduced accidental pregnancies which were a decent amount of the fertility rate. Then we attacked teenage pregnancy with a vengeance. In 1957 it was 96/100k teen women had babies, 62/100k in 1991 and now down to the current rate of 11/100k. The postponement of births expands the time between generations which compounds the problem. An 18 yr old could have a baby that has a baby at 18 before a 36+ year old mom has their first child.

All this leads to exponential decay of humanity. In the near term we don't have to worry about extinction but we do have to worry about the pyramid schemes we have to support non-workers (like social security). This will all play out much sooner in Asia where the TFRs can be half of the US/EU. Imagine due to China's one child policy a young working person will soon have to support 2 parents and 4 grandparents somehow. There will be some kind of reckoning and some of the speculation around what it will look like is quite grim.

> A lot of the decrease is also correlated with access to birth control which drastically reduced accidental pregnancies which were a decent amount of the fertility rate

This was my first intuition.