Why is Lena Younger the head of the family while there is a 35 year old in the house?

Although Walter Lee is old enough to be the head of the household, he is unfortunately not responsible enough to take on his role as head of the family. His mother, Lena, as the matriarch, wants her son to assume his responsibilities, but she doesn't want him to simply say that it is his role..... she wants him to take on the role and be what he claims to be capable of. Walter's actions belie his words.