Why were sons especially valued in Jewish society?

Because they could carry on the family name when they got married and had a son. They were also valued because they helped the father in whatever business he was in and were allowed to get educated before the women were. The sons could be trained in a number of skills and fathers needed them for this advantage.

Because you cannot technically be Jewish (under Jewish law) by birth unless your father is Jewish--so technically the more males there are, the bigger the religion will get.