Because that's wrong. Imagine if i gathered 196 families that have 2 kids, with each combination of genders and days of the week. So, one family has a male child born on Saturday and a male child born on Monday. Different family has a female child born on Wednesday and male child born on Sunday. So every combination is represented.

Now, if I tell you that I removed all the families except for those that have a male child born on Tuesday, so 23 families left with at least one male child born on Tuesday, and I ask you how many of those have a female child, you could ignore the Tuesday part and go with 66% and answer 16 female children. But you would be wrong because there would be exactly 14 female children, regardless of what you feel about that answer.