There's always "too much secret sauce". I remember a time when CryTek published papers about how they implemented things, now they don't.

There's always in-house ways to deal with graphics drivers and certain effects. Remember how Source 1 was the only engine which was able to render HDR with measly ATI 9600XT, without a performance penalty most of the time?

You carry this know how in evolved form for generations, and it gets buried under as the foundation of the new things you're building on top of it.

This is what I remember from a friend who implemented his own game engine and created a company for it, and half the woes were making the graphics drivers and processors behave as they said in their manuals.