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.
A commercial quality game engine is a huge project, and as such, requires a huge projects worth of time and effort to build. Same as any other huge software project (which all have countless abandoned open source attempts). Remember that most open source projects start of tiny (one developer is common, certainly very few start off with more than a handful) and work on it in their free time, while commercial teams like Unity have hundreds of peolpe working on it full time.
The reason they are abandoned is typicaally that 1) the developers, who are doing it in their spare time, don’t have the time anymore; or 2) they get frustrated at the amount of work/lack of contributors/lack of interest and stop working on it; or 3) they simply lose interest and work on something else.
Its not lack of ability, its lack of time/motivation/interest to keep working on it for many, many years.
The graveyard of hundreds of abandoned open source game engines begs to disagree with you. I agree that it seems absurd, but you really can't underestimate the amount of man hours put into these things. Unreal has been under constant development for over 20 years.
It's the same as flight simulation. Why is there only one viable flight simulator (X-Plane) on the market? It's because that kind of coding is simply beyond 99% of developers, and it takes a huge well funded and focused team to make something state of the art.