No installer, but site is there?
As André pointed out, you only need to run the installer if you need Craft to create all of the necessary tables in a blank database.

If you've dumped your database from dev/staging and imported it into production, there is no need to run the installer, because the tables are already there.

If Craft sees someone try to load the installer on a site that already has a populated database like you have, it will return a 404.