So that the closed source people can't copy open source and sell them as their products and if they do then open source people have the "legal tool" aka the license to tackle them legally.
then it would simply mean it is not open. When someone creates something, by default it's "closed" and it's their property. If you take that code and use it when there's no permission, that's stealing. If you want to "open" up your code so anyone can use it, you should explicitly tell people they can take it for free. Meet license.