How add SSL/443 to Apache server without virtual host?

By default, in CentOS, there is a file used by Apache/httpd located at /etc/httpd/conf.d/ssl.conf. This file is read in as a configuration by Apache along with the "httpd.conf" file and anything in it takes precedence over settings in httpd.conf.

That file (again by default) contains a Listen 443 directive. You cannot call that directive twice (as it will say it's already been bound to that port), so that caused the conflict. After removing that, it works.