This is by design and intentional. WordPress rewrites have become increasingly complex over the years, and many plugins utilise the page
endpoint for a page (usually with a template and custom query) – redirecting introduces a potential world of pain.
Long story short, it doesn’t matter anyway. WordPress adds <link rel="canonical />
for pages, so no need to worry over duplicate content.
Update: For localised situations where you want to disregard the potential risks, this will canonicalize all page URLS – note that it does not check if a page is actually paginated (i.e. with the <!--nextpage-->
quicktag) and will break this feature if you use it.
function wpse_199180_canonical_pages( $wp ) {
if ( ! is_admin() && is_page() && isset( $wp->query_vars['paged'] ) ) {
wp_redirect( get_permalink( get_queried_object() ), 301 );
exit;
}
}
add_action( 'wp', 'wpse_199180_canonical_pages' );