Sending mails with wp_mail() to multiple recipients using wp_cron and dynamic email body data

Here’s your problem:

        $body = file_get_contents( get_stylesheet_directory() . '/templates/email/event-creator-notification.php');

file_get_contents literally just gets the contents of that file. There’s no PHP execution. Instead, if you want to get the output of something as a variable, use output buffers, e.g.

ob_start(); // everything echo'd after this gets put in the buffer
get_template_part( 'templates/email/event-creator-notification' );
$body = ob_get_clean(); // get the buffers contents and stop output buffering