Hehe, this is actually a trick I use for testing a site after local development and before going live. People simply see a html file, but the ones who know can access the site by typing index.php.
I use simplix, does this mean that I'm a simple guy ?
When you install Joomla!, many files and folders are installed: index.php, index2.php configuration.php, /components, /mambots, etc.
The trouble, in your case, was the presence in the same folder of index.html. If you create your site in html (with Dreamweaver, Golive, etc.), you would keep there index.html. But for Joomla!, index.php is needed.