Why is everyone in marxism equal?

Marxism does not say that everyone is simply equal. Marx and Engels thought that people should be equal (socially, economically*, politically, etc) and that people should all enjoy a happy life free of exploitation and war and so on. But Marx and Engels siad that this will not come about by reforming capitalism, which systematically creates and spreads inequality, but will be brought about by people as a whole orgainsing and overthrowing capitalism and replacing it with a system that will create and spread equality, socialism. Though there will be still inequality under socialism but it will take socialism's maturing and development to facilitate the development of communism in which they dream of a world free exploitation and so forth will be realised.

*Though bear in mind that Marx and Engels did not think that everyone should be paid the same or anything like that because everyone is different. Some have large families and so on, if they got the same as a newly wed couple or something there would be marked inequality.