What do you consider a Game Engine?
For all purposes, a Game Engine is defined as
a standalone tool which main purpose is to create games without the need of any assisting tool
. This is by no means an universal definition, and even this definition had to be stretched a bit so we could add engines such as Fantasy Consoles. Even so, that's out general rule-of-thumb to add Game Engines on the website.
Why do you only accept Open-Source games?
For a couple of reasons:
The main purpose of listing games is educational.
We want Game Developers to use the existing open-source games as reference to answer questions such as "how to I organize assets in this Engine" or "how do I program using this script language".
It's a reliable way to validate the game.
We want to make sure every game was made by the engines it refers to.
It gives visibility to open-source games.
We want more open-source games, and we want more people knowing about them.