Life is lonely.
All Cubone know this.
Hatched beside a deceased mother whose last act was to provide a chance for the following generation, each Cubone learned that Death was necessary and inescapable.
Every Cubone don their mother's skull and carry her bone into battle to defy Death for as long as possible and display their courage.
Perhaps they would find a being to make their life less lonely but in the act of creation the females remember their mother and the males know no other way of living.