Define pages are PHP files, and can process PHP. Product descriptions are HTML content stored in the database, like ez-pages. It has been discussed in the forum how to enable PHP for ez-pages, but pointed out that there are security risks; a SQL injection attack could then run PHP and do even greater damage.

You can add an include to tpl_product_info_display.php to bring in your content files. Look at swguy's boilerplate tips, available through his website and referenced from his sig and possibly forum posts. This is intended to be used for information that applies to many products, but you could just as well use it with a different file for every product and edit an HTML file when you want to change it.