WP_Query only parent categories

query.php

<?php
namespace App;

use \WP_Query;

$categories = get_categories( ['parent' => 0] );
$ids = array_map( function( $category ) {
  return $category->term_id;
}, $categories);

// Or with arrow function as in PHP 7.4
// $ids = array_map( fn($category) => $category->term_id, $categories );

$posts = new WP_Query( [
  'category__in' => $ids,
] );