For all of the (well-founded) anxieties about overpopulation and resource allocation on Earth; it should be noted that birth rates have fallen sharply almost everywhere on the planet. They continue to do so – even in places where birth rates were already below replacement levels –, and that this is the case in practically every country, except for a few states of very low development levels in the African continent. Earth’s population may plateau and start decreasing sooner than we expect, although this may happen too late to mitigate the effects of climate change and our impact on the planet’s livability.
Some environmental or lifestyle factors do explain why some people can’t have children despite wanting them: there is also somewhat of a rising fertility problem in many rich western countries. This is mostly due to people’s choice to delay childbearing until much later in life than they used to; and by the time they are hitting their late thirties, people with specific health or lifestyle issues (from diabetes to heavy smoking or substance abuse) may find themselves in a situation where they want to have a child, or more children than they currently have, and cannot. These cases however are still exceptional.
The much simpler answer to the titular question is “because now they can choose it”. Most people today, especially women (who historically didn’t have that option), get to choose if they want children and how many. Many others are choosing to have only one child – which over time still amounts to a population decrease, since “one” is below the replacement level. It rather serves to ask the opposite question: why did people in the past have so many more children? What did bigger families mean to them?...
One of the more obvious answers is that there were no reliable contraceptive methods – children happened simply because pregnancies were more difficult to avoid consciously. Men and women married younger and therefore start having children earlier, often amounting to many more offspring over the course of their lives. A large part of the answer was the lack of the possibility to choose that nowadays exists. Another part is that most societies were agriculture-based and with no state-financed social security or retirement plans: having many children meant more hands to work on farms and a form of “retirement insurance” where the children would support their older parents. Models like these are still the case in the poorer regions of many developing societies.
Having as many children as nature would decide, used to be simply a given – many children were taken to be the inevitable event to most married or cohabitating couples. Times are rapidly changing; the possibility of choice being one of the many newer benefits. Many people of a traditional mindset still consider having children something of an obligation and regard it as “selfish” not to have any. Others would strongly disagree, as they offer their – often very well thought-out – reasons not to have children, or to have them later in life: financial distress, reduced possibilities to offer the child a secure life, acknowledging that one is perhaps unfit to be a parent, etc. Ironically, a lot of those judging the childless present their own reasons to have children, which sometimes sound – surprise! – entirely selfish: “I want children for my legacy / my genes / my motherhood or fatherhood experience / my family continuity / to cure my possible loneliness / to make up for my inexistent pension plan”… All in all, the possibility of choice can still be viewed as yielding net positives. The fact that people wait to have children later in life, and that access to contraceptives is so widespread, means that many more people today than before are having children because they truly, consciously want them, and are better prepared to provide for them.