There was interest in having it and then it seems the feeling was it's complex.  the Caching API allows modules to implement a cache.  However, the real benefit of file based caching is 1) Use lightweight webservers threads to serve files 2) Don't need to bootstrap Drupal 3) Don't need an expensive MySQL thread.

Keep asking for it and it's more likely to get in since the code is available to be patched against head.

Kieran