Infant and child mortality was very high. Empirically the high birth rate did not always lead to overpopulation - overpopulation waxed and waned despite the persistently high birth rate.

The sci-fi dystopias about overpopulation never really come up with a mechanism by which the birth rate remains high despite low infant and child mortality and low economic need for human labor. It always seems to boil down to "people are controlled by irrational instincts".

There's an old cemetery in Redwood City. Very roughly speaking it was active for about 100 years between 1850 and 1950.

There are multiple grave markers listing children that died under the age of 10. Like a family had a 4 year old die, then had a baby 2 years later and then 3 years later buried that baby in the same grave as their older sibling.

This was well before birth control became widespread. When you can decouple the fun part from the responsibility that comes with it, you do so. Hence, why the fertility rate has been falling below replacement around the developed world.