[1] https://en.wikipedia.org/wiki/List_of_countries_by_total_fer...
[2] https://data.worldbank.org/indicator/SP.DYN.TFRT.IN?location...
[3] https://en.wikipedia.org/wiki/Sub-Saharan_Africa#Demographic...
1) People used to have lots of children because most died in child birth and also over a period of time due to infectious diseases, accidents, etc -- lack of current level of healthcare. There were only a few children that survived.
Medical care increased the survival along these three dimensions: vaccinations, public health programs (clean drinking water, sanitation, etc -- which prevented communicable diseases like cholera, plague), and infant mortality.
2) TV. If there is no other form of entertainment, sex is the only thing to entertain yourself, and without contraception, it inevitably leads to births.
The real misery in the life of man is boredom. Simple desire for sensation. With TV (and now phones and nonstop junk consumption of content), there is a a slow drip of dopamine where you don't get bored. The accidental days you have sex, there are a range of birth control mechanisms.
3) Contraception. Even if people have sex, it is consequence free. Sex is fundamentally biological, biology cares about nothing more than continuous replication, but thats hijacked by contraception.
4) The power of defaults. Marriage and children were the socially constructed defaults for women. Women are required to take care of children, while men's work is considered meritocratic, paid in wages. There is a number, men -- $xx, women -- $0; a woman's work is worthless because it can't be quantitied. In a data driven culture, women are noticing and getting jobs worth $xx and more.
5) Status and evolutionary biology. Women can now make $xx. They need a partner that makes at least $xx++, is 6.5 ft tall, built like Chris Hemsworth, has a number of degrees (academically higher status -- not a plumber!), has legacy wealth and doesn't have to go to work so the man can be there for the woman all the time without having to deal with pesky things like going to work (like Edward Cullen).
6) Media telling women that they shouldn't accept anything less than the absolute ideal of a man. Media has its vested interests. People in families (either happy or unhappy) watch less content, they just get busy with a lot of things.
7) The super rich working extra hard to squeeze everything out of everyone. Two jobs (for a family) was supposed to give greater financial security and freedom. Instead, at least 2 jobs and overwork became the norm. And the loss of job security, you never know when you'll be out of a job and loss of healthcare, housing and everything else.
8) Rentier capitalism. The super rich monopolizing and creating permanent structures to extract wealth from absolute necessities. The basic human infrastructure that Govt should be helping with: housing, health, etc.
9) Idiocracy. Thoughtful people making a decision not to have children -- because of the uncertainties of life and impossible challenge of raising a child, raising a child right, having the time and making the world a tolerable place for their children.
10) Marriage (and hence kids) used to be an economic arrangement to make things work as a family unit when there is extreme scarcity. People made sacrifices. Younger generations grew up in very comfortable environment, there is no forcing function to make any kind of sacrifices, let alone tolerate even the slightest discomfort.
Also, humans went from 100 million on the planet and will be 10 billion soon. I don't think there is any great danger of us running out of people.