Define_main_page and template override

I've tried looking, but this question doesn't quite seem to be addressed.

And then again, maybe it's just me. I'm working on a new site and I still don't understand why the define_main_page content has to be edited in the english directory, and not in my override directory.

Someone please illuminate me!

Thanks,

Ray

The define pages structure works like this:

The master file for english is:
/includes/languages/english/html_includes/define_xxxxxxxx.php

The override files for english are located at:
/includes/languages/english/html_includes/YOUR_TEMPLATE/define_xxxxxxxx.php

and not in my override directory.
Addendum: You can have an override dir anywhere there is a "classic" dir and then only for files that are directly under this location