c++ templates

Quick answer : because current compilers do not support the ''export'' keyword that would let you do just that.

Longer answer : It is more or less the same reasons as why you need to put both the code of inline functions and declaration of class member variables in header files.
When you instanciate a template, the compiler needs to find the code it has to adapt to the required specialization. When the compiler processes file A (which includes header H), it doesn''t know anything about the code in file B. Remember that variables have to be declared extern to tell the compiler that they exist, somewhere else, and will be resolved at link time. Function prototypes create the same kind of placeholders.
But when you use a template, you need the code NOW, because you''re going to ''fill the holes'' left by the template parameters. Same as for inline functions, same as for information on the size of a class (hence the member variables).