Why are java header files not used in C++ ?

There is no real need in C++ to have the class definitions and declarations in separate java header files. It just means that you could, at least back in the C days, do the parse in a single top-bottom scan of the code. On machines without random access storage this was a big deal!

Having headers also allowed you to publish the interface to your code library by supplying the header without needing to reveal the source code. Unfortunately in C++ you also have to reveal the private data members which has led to solutions like the horror of pimples.

There have been attempts to make a C++ environment where everything was stored in a database type structure and there are no files but it didn't catch on.