Game engine development is so ridiculously hard. The industry has plenty of little libraries that do some significant upfront lifting, but there's just plenty out there that isn't done for you. Stuff you wouldn't even think about until you realize you want a feature that isn't there. And the further you progress, and the more you learn, the fewer references and prior art you can turn to, because the combinatorics of finding someone who has built up all of those areas of discipline becomes exponentially harder to find.

There are still proprietary technologies today that are decades old that the free and open source community has not recreated simply because for all of the people that work in this space, open source and commercial, there are just not many people at the high end of the scale working on those interesting problems. Those problems are the ones that generally become commercial solutions because no one else is doing it.

After over 14 years of development, I would love to hear what little big problems like this the authors came across.