The reason isn't because it is more difficult or people enjoy having kids less (like the parent). Its because children used to be a way to provide security for your family and community. More hands to help or sent off to earn money younger for example.

Now with smaller family units and less community interaction they represent a risk to security, mainly financially.

Children still provide security for the family and community, it's just that it's done in a way where the people creating the benefit and the people realizing it are almost entirely different. This is due to the way social security works. Creating a tragedy of the commons where everyone wants more kids but no one wants to be the sucker that has them (from the financial viewpoint, obviously not from others). Obviously this sort of communism where society takes the profits in form of social security but provides almost nil (except some pittances in property taxes for school) for the investment creates a collapsing system.

Having children for yourself provides more security for all the other families but barely anymore for your own. Meanwhile you bear most the costs and everyone else bears very little. So the incentives are totally reversed, and even worse the coupling between investment in children and payoff is cut which means the people in the best position to help their kids be successful are the least incentivized to do so.