The developing world has birth control but, in large parts of it (e.g. most of africa), most people aren't using it. This is for economic reasons. In these economies children are your pension fund. There's no RRSP funds or old age security payments. Just children who (hopefully) look after you in your old age. If you have only one or two you will either be an unreasonable burden upon them or simply be abandoned to starve.

The economic support system for seniors in the developed world is currently under a lot of strain too. In countries where pension funds are funded from current taxes the baby-boomer retirement is going to be a tremendous drain on society. The baby-boomer generation should have tucked away a nice huge fund to pay them in retirement as well as support the greater demand on the medical system. Instead, they racked up huge amounts of public debt. Massive amounts of immigration and raising the retirement age have been the stop-gap measure of choice so far, but the developed world's support for senior citizens could easily revert to that of the developing world's without much warning!

So, yes, children are expensive, but for most of history and in large parts of the world today they are a necessary investment for retirement. In only the developed world of the past century have financial pressures favored fewer children, and that could easily change.