Do you think that it's sacred knowledge? Or just nobody cared enough to understand how ffmpeg works, because those guys are doing good work already?
Also I don't really think that it's fair to claim something for everyone in the world. There are plenty of developers, forking some project and working silently upon it inside some big corporation. I'm sure that there's some chinese guy chuckling on your words as he writes some assembly optimization for H.264 for some rare architecture.
It's just that one guy in Nebraska situation is kind of local optimum and everyone is more or less OK with that, at least for the time being.
Your comment is absolutely spot on, I've known people forking GCC internally and beating stock GCC with 7% to 10% on an obscure architecture -- and legal prohibited them to share the patch upstream.
I've seen such things several times in my career and it made me very skeptical of people generalizing, like your parent poster kind of did.
Not just inside big corps either. Sometimes they just do not like the upstream devs and are not going to share.
One project I watch one dev in charge of the project is seeing comment after comment in the project is 'I will just keep these changes to myself then, closed' and specifically because of him. Lots of devs are keeping their stuff private.
I personally have done this in another project. I got tired of arguing with one dev over 2 lines of code. So my private version works. The public one is broken in that case. That was 2 years ago. It is still broken publicly and the forums for that group are telling them that...
There's about 4 people that really understand how ffmpeg works, and thousands of people that write stuff that runs on the top.