Most people are stupid and incredibly short-sighted, most of the time. If pre-modern serfs and slaves had access to a fairly reliable, inexpensive means of avoiding contraception, none of us would exist and the human race would be caught between Malthus on the one hand and demographic collapse on other.

It's obvious that future generations are a net gain as far as the advancement of knowledge or our material well-being go, but the immediate cost of having and raising a child is shouldered mostly by their family. Incredibly few people (both now and back then) have children for the greater benefit of society.