What's the point of having libraries?

Question

For example with <cmath> in C++ you have the ability to use extra mathematic numbers. To clear my question what I mean is why a language can't have all these libraries as standard and you have to enable them. Is it related to compiler's speed? or what?

Answer 1

Main reason is modularity and ability to write your own library for othet developers. There is no need to have every library in your application.