functions.php

<?php 

function add_pages_to_dropdown( $pages, $r ){
    
    if ( ! isset( $r[ 'name' ] ) )
        return $pages;

    if ( 'page_on_front' == $r[ 'name' ] ) {
        $args = array(
            'post_type' => 'post'
        );

        $posts = get_posts( $args );
        $pages = array_merge( $pages, $posts );
    }

    return $pages;
}


add_filter( 'get_pages', 'add_pages_to_dropdown', 10, 2 );