Because it's common sense that this kind of thing is ALREADY implemented when needed.
- If you have a PostgeSQL database running, from your firewall you will only allow your php/apache/whatever server to connect on
tcp/5432. You will also BIND your application to listen only at the desired ip address, and you will also configure PostreSQL to allow connections from your application server.
- If you have a *NIX server hosted at a cloud provider with ssh enabled, and you have some valid IP adresses at your company, you will only allow those IP's to reach your cloud instance. You could also implement port-knocking techniques to make it less easy to one scan your services. Other layer of protecion are ssh-keys.
The Principle of least privilege also applies to network. But on THAT GeoIP case, there was a special need. Maybe he need to keep an application available only to his country ip addresses.
It's not done everywhere because it's not needed on every situation.