For the simple cases, a language package manager is easier to write and makes things easy. However, simple cases aren't at everything and for the more complex cases, such as what C and C++ is commonly used for, a language package manager is not complex enough to handle all the little weird special cases and so it would be a bad idea.
Those who use a language for something complex will not use the built-in package manager. Realistically though, very few people are writing complex software and so this objection, although real, is probably a pedantic thing that doesn't affect you.