There are several reasons not to reuse 3rd party OGC. Some of them good, some of them not-so-good. But all of them taken together can be a reason not to do it.
1) There is no way to know that the OGC in the 3rd party book is licensed correctly. Or that the 3rd party has sufficient rights to put the material under OGC.
2) Some OGC is hard to identify as OGC and so while you might enjoy using such-and-such a rule from some book, that doesn't mean its OGC can easily be reused.
3) Reprinting OGC is a dual-edged sword. It takes up space in the book. Some people will have the source material and will feel they are paying for the same material twice if you reprint the OGC. Others won't have access to the original and if you don't reprint it, the new stuff is unusable. You can't please both camps so what do you do?
As a corellary to this, if you don't republish, the only people buying your product and using it fully are the 800 people who bought the other product. So instead having a small market for your product you now only have a tiny market (diminutive market?).
4) Ego. Most 3rd party pubs became 3rd party pubs so they could publish "their stuff". Reusing someone else's stuff is counter-indicitive of the personality type. Then there's the whole, "this is good but my way would make it better" problem
5) The license makes it difficult to say stuff like "Compatible with Ritual rules from Necromancer Games' Relics and Rituals". So even if you are proud to be compatible with older rules, you need to arrange licenses with the creators to say so. This legal hurdle is usually not difficult, but over time, it could turn into a big legal licensing file you have to maintain to ensure your compliance all around.
Scribble said:
I'm hoping that now that the D20 market is slowing down on what "needs" to be done, companies will stop trying to rush out and be the first with a set of rules for doing something...
Um, nothing "needs" to be done at all. And eventually RPG customers learn this and stop buying. You really only need the core 3 books to play D&D for decades. Everything else is just icing.