Asking wet nurses to feed their children was the most popular in Ming and Qing Dynasties. At that time, it was almost a custom to invite a wet nurse. Until now, it must be because mothers can't breastfeed, but there were other reasons at that time, because hiring a wet nurse was a symbol of status, not just because biological mothers can't breastfeed.
Because most people can't afford a wet nurse, only some big families or bureaucratic families can afford it. It's a shame to have a wet nurse at home. On the contrary, if the wives of a large family breast-feed their children without a wet nurse, outsiders will joke that they look like the wives of a large family, but they are actually wet nurses. Otherwise, they will say that the family is too stingy, even worse than the wet nurse.
Not only that, in ancient times, the status of women could not be compared with that of men. A woman's duty is to serve her husband well, especially some big families. Men can have many women at the same time. In order to compete for affection, women will not breast-feed even if they have children, because breast-feeding may lead to body deformation, delay time and give other wives an opportunity. No woman will give up fighting for the favor of her children.