In many languages such as C, C++, and Java, the main method/function has a return type of void or int, but not double or String. What might be the reasons behind that?

I know a little bit that we can't do that because main is called by runtime library and it expects some syntax like int main() or int main(int,char**) so we have to stick to that.

So my question is: why does main have the type signature that it has, and not a different one?