Not exactly. You needed to buy a WordPress Theme. ThemeForest is not a marketplace dedicated to WordPress, it has themes for various platforms, and even standalone HTML templates (which is what you bought).
Yes, a “site template” is HTML. If you click on “HTML” at the top of ThemeForest you’ll see the category where you bought that item. It does not come with any PHP code for integration with any platforms.
“Themes” are templates with PHP code on top of them for integration with a platform such as WordPress, Drupal, or Joomla. If you want to convert a site template to a WordPress theme you will be expected to write that code yourself.
They’re promoted just as equally as WordPress themes, Drupal themes, Joomla themes, etc. That’s because they are one of the major categories of TF, and plenty of people do search for and buy them (including myself).
Unfortunately that’s just a misunderstanding on your part, but you’re not the first person to make this mistake nor will you be the last. Some authors try to prevent this by clarifying on the item description that “This item is NOT a WordPress theme!” but to be fair the item you bought didn’t have a single mention of WordPress on it so the item itself is definitely not misleading.
Also, HTML templates are required to be a “complete package” here - they must include a full set of pages and be fully designed.
These templates are not worthless at all. Someone with fairly basic PHP knowledge can purchase a template for far cheaper than a WordPress theme, and turn it into a full-fledged online store very quickly. That’s why these are on sale, because they’re useful to a lot of people.
Unfortunately it looks like the template you bought doesn’t have a WordPress version. You can hire someone from Envato Studio to convert it into a WordPress theme, or find a different item in the WordPress section.