I'm made a pretty sexy engine (if I do say so myself) for my own game, but after months of release it only hit 100 players. 20 of which were bots. So it was a failed process. I was gonna push the project to github to let people learn from it, but I completely forgot to it.
Point being, I saw these engines and said. What the hell will I learn buying someones stuff. I took a few months and made my own. You learn a lot more and fixing bugs in code you wrote is a helluva a lot easier than learning someone elses code, which usually sucks and isn't well documented.