Why isn’t is_page_template() adding a body class?

The is_page_template() function is the incorrect function to use in this case as it checks against Page Templates and single.php is just a normal template, not a page specific one, usually meant for posts.

The function you’re probably looking to use instead is is_single( $optional_posttype ) which will look for singular view of a post type, post by default.

if( is_single() ) {
    /** ... **/
}

You could also check against the basename if you really wanted to:

global $template;
$template_slug = basename( $template );

if( 'single.php' === $template_slug ) {
 /** ... **/
}