It could just be different interests. The kind of person who makes a game engine is a technical optimization-focused tech-focused person, sort of like a mechanic. In order to make a game, you have to deal with softer concepts like "is this fun" which is more like a designer/artist. Game studios need to bring these people together, but in the FOSS world the mechanics are happy to spend their time building an engine that runs beautifully without concerning themselves with the art side of things.
I just need a few more years working on my 4D non-euclidian voxel MMO engine before I can make my game!
A tool with a vaguely defined goals and no stakeholders is easier to make than a tool that must meet certain goals as defined by stakeholders.
Well... the project calls itself a game engine. It's really not out of the world to make the assumption.
Making a game engine is a fun and highly-engaging means to learning high-performance programming. Yes, it would be better if you also were able to invest enough to ship a game. But, don't let the infeasibility of that goal stop you from learning and having fun.
I agree there is fun and learning to be had, but just note they are very different activities.
somewhat past optimizing the frame count of an entirely empty scene.
on that matter : is it a game engine if there isn't a game?
I totally agree with other comments though -- if there is no pressure to meet specific metrics or accomplish certain things with the product then there is no real pressure to improve past a window or framebuffer drawn to video, just declare it's a game engine that makes a million FPS and throw it on the portfolio.
game engine work gets tough (and rewarding) with 1) goals and 2) constraints -- without those two it's more or less just spherical-cow style work that is too ambiguous or vague for real application.
The skillset and ontent is also just different. You don't see games on github (publicly) because they are being made for sale. Very few engine projects are serious commercial projects. I think I'd be safe to say that a commercial engine is harder than making a commercial game. Especially since Unity and Unreal have mindshare and are free to start and learn with.
I usually look for games on websites like Itch.io. You might want to try that if you're having trouble finding websites that have games on them.
OP is trying to make it seem like some sort of failing, where people aren't cut out for game dev or something so they just make game engines instead. But I can guarantee that anyone who can create a game engine can also create a game. They just choose to do something they enjoy more.
When an engine becomes useful is when it has to make a game. All your abstractions tend to get rearranged and hard decisions are made.