Why did they boys/girls get tortured by their fathers in ancient rome till they were 6 years old ?

Because, it was their culture, or part of it. boys were raised to be soldiers who were ruthless and who had to fight while the women stayed home, but they had to
defend themselves in some sort of way.. so it taught them how to fight and be tough, and how to protect themselves from the men who came back from war who would try to attack them.