Not MOST mothers but SOME mothers. There are mothers who prefer sons because sons AREN'T as threatening to the feminine dynamic as daughters would be. There are mothers who are IN COMPETITION w/ their daughters. They view their daughters as the better &/or younger version of themselves. They see in their daughters never had &/or lost opportunities. This becomes more evident as daughters enter adolescence.
Such mothers are more comfortable with sons as they will never compete w/their mothers. They see sons as more soothing psychologically, even psychically. There ISN'T the feminine mano o mano that is oftentimes experienced in the mother-daughter relationships. These mothers of sons have the female dominance of the household & DON'T have to vie w/their sons for such dominance as they would if they had daughters.
Some mothers see daughters as quite problematic because the daughters will probably surpass & eventually gain some type of dominance, even preeminence in the family & life dynamic, particularly as the mother becomes older. Remember, the daughter is view as the younger/better version of the mother & some mothers view this very negatively w/ some becoming highly soul devouring, even destructive in the dealings w/ their daughters in order to keep the daughters "in their place" which is ALWAYS SUBORDINATE to their mothers.
Also, there are mothers who view their sons as manageable. They contend that sons are not melodramatic as daughters are. They maintain the sons are more stable & consistent in their wants & desires. They even assert that sons are more forthright, saying what they mean. They assert that daughters as girls/women can be quite erratic, even unstable. They may even contend that daughters can be quite unmanageable because the latter are considered to be "in competition" with them for that feminine throne. They furthermore view their daughters as typical females, using subtlety/underhandedness/manipulation to get what they what from them & other family members.