cPanel, Softaculous Drupal installation does not include install.php
The Softaculous App Installer included with cPanel provides a very easy means of installing Drupal in your cPanel account. When the installation completes, softaculous removes install.php. This is by design. Normally install.php is not needed once Drupal is installed, and removing the installer protects your website from some forms of attack.
When installing Drupal with Softaculous, install.php is not included.
Advanced users might have a need to use install.php in some scenarios, such as setting up a multisite Drupal instance. If you require a copy of install.php, you can download current or previous releases of Drupal from drupal.org. Once you have the release you want, you can extract it on your local machine and upload only the install.php file to your cPanel account. This file will normally be placed in public_html, but may be deeper in the directory structure if you have installed Drupal in a subdirectory.