Actually they can. It's part of the reason why a lot of capitalist nations are seeing major problems with population stagnation and possibly shrinkage.
The problem is markets don't care at all about society. If they can require that every member of a household has to work and extract all their money as efficiently as possible, then they leave little room for society to have families.
Capitalism is geared towards minimizing workers' free time. And, unfortunately, free time is how babies get made and kids get raised.
That is where western capitalism is failing. Shouting louder and young adults to pull on bootstraps harder isn't making them have kids in their studio apartments.
South Korea and Japan are 2 examples of this train-wreck that's coming for the US and other nations.