Wrong - you're assuming all apps are served over the web, the network firewall is not going to care what port you run on an app on your machine - it only applies to traffic in/out of the network. You better go tell Express.js and all the apps that use 1337 by default they're all doing it wrong, and you know something they don't know.

My external web services are of course served over HTTPS (8443) to external traffic, but there's nothing wrong at all with using 1337 for client-side apps, as I mentioned with Kdenlive (a video editing software that runs on my desktop).