There are cases where it's difficult or impractical to open source a game, e.g. if it includes assets or code that are licensed (so sometimes you will see the engine for a game open sourced but not any of the content), or maybe the code is really messy and I don't want the world to see it, or I'm still grasping onto the faint hope of making a living or gas money from this stuff. Anyway, I'm all for open source and I use a lot of it (if I make another 3D game for myself, it'll be with Godot), but rather then tell others they should open source their work, I think it's better to lead by example and open source your work to encourage others. There's nothing to stop you from uploading code or projects to itch.io, but for source code I'd suggest deploying something like github so that people can browse, fork, and submit improvements.