There’re are some difference between a codec or OS with a game engine. One is the user base. I think there’re simply not enough game developers to extensively use the open source engines, every single computer needs an OS and in most cases needs a codec. But not everyone is a game developer. Another one is requirements I guess. For OS and codecs, I think the requirements are much more defined and manageable, but for a game engine the user uses it to create something else, hence the capabilities needed from the game engine may change wildly per project.

And yet those codecs still aren't in hardware devices or widely implemented in all browsers.

I wouldn't cite open source codecs as a success story ...

A GUI toolkit is probably more on point and of the correct scale, and those also demonstrate that without significant backing from somewhere, progress doesn't get made at that level of complexity.