Humans need to be ethical because they live in groups. (Groups are families, villages, towns, cities, clubs, schools, communities, workplaces, etc). For groups to run well, they need a certain amount of discipline and rules. Generally, groups make rules they agree with, so that they can live together without too much trouble.
Ethics means sticking to agreed rules. It means abiding by certain principles that ensure peace within groups.
If people lived all on their own, ethics would not be necessary.