Question: Why isn't C/C++ called platform independent like java when the same source code written in C/C++ can be made to run on different operating systems by different compilers, just like JVM is used in java.

Isn't different compilers and JVM doing same thing and achieving platform independence.

Answer: 
In case of C or C++ (language that are not platform independent), the compiler generates an .exe file which is OS dependent. When we try to run this .exe file on another OS it does not run, since it is OS dependent and hence is not compatible with the other OS.
The role of JVM in the independent platform is that it acts as a virtual processor. when we used c/c++ to compiler different processor converts the source code into a different binary pattern that's why there are not platform-independent.