FindBLAS: Why no BLAS_INCLUDE_DIRS?

FindBLAS defines

``BLAS_FOUND``
  library implementing the BLAS interface is found
``BLAS_LINKER_FLAGS``
  uncached list of required linker flags (excluding ``-l`` and ``-L``).
``BLAS_LIBRARIES``
  uncached list of libraries (using full path name) to link against
  to use BLAS (may be empty if compiler implicitly links BLAS)
``BLAS95_LIBRARIES``
  uncached list of libraries (using full path name) to link against
  to use BLAS95 interface
``BLAS95_FOUND``
  library implementing the BLAS95 interface is found

Why not BLAS_INCLUDE_DIRS?

Related question on Stackoverflow: https://stackoverflow.com/questions/39748767/find-blas-include-directory-with-cmake