Why include iostream despite including fstream though fstream is derived from iostream?

Till a while ago, I was using a C++ compiler with old standards(TurboC++), where, on including fstream.h, including iostream.h became needless(in order to use cout/cin, for example).

Now, when I switched to newer standard compiler, I find that I need to include iostream even after including fstream, despite the fact that fstream is derived from iostream.So, I want to know why it is needed to include iostream with fstream and increase compile time.