Why isn't my theme processing SASS?

It's my understanding that Wordpress now processes SASS files but for some reason mine don't work.

I'm trying to call it using wp_enqueue_style directly in header.php via the head tag:

?php wp_enqueue_style('stylesheet', get_template_directory_uri() . '/style.css', $ver = false, $media = 'screen') ?

Instead, the file gets called as a normal CSS doc.

I attempted to rename style.css to style.scss but then the file doesn't load at all, so I guess the sass pre-processor just isn't firing.