Anyway, I actually don't think they do. Because they own their own OGL stuff, they aren't actually using the OGL when they use their own OGL material.
It's not like a first publication deal, where you have to include the first published source of the content. Just because you publish something as OGL, doesn't mean you give up your rights to it, you are just licensing it to others. Like WOTC - they published the bulk of the PHB as OGL material, but at the same time, they publish the same stuff over and over in non-OGL books.
Only if their OGL material is derivative (and by that, I mean using their text) of someone elses material, do they have to include that.
Furthermore, who would take issue with such a thing? The 3rd parties in question could never prove that the original company didn't have a version of that book sans any 3rd party content. Because all they would need is a PDF or manuscript or something without it (which they probably do have, draft versions).
Not to mention, even in practice, I don't know of any company ever taking action on OGL violations, despite misuses.
For instance, something like this sorta happened, but the company in question used material that was actually from another company, but didn't credit in the second product they used. Mongoose used part of their Noble class power class in the Conan RPG, but didn't credit the company that much of the Noble power class was derived from (Fading Suns d20). Even though some of the text is identical in Conan RPG as Fading Suns d20, AFAIK, Hollistic never did anything, even when that same text appeared in later Conan products.
(Though it's sort of karmic for Hollistic, as they literally borrowed stuff from the Star Wars d20 RPG. They rewrote the sentences somewhat, enough to get by legally, I'd guess but kinda tacky)
(And yeah, WOTC took action vs Fast Forward, but that was for them using stuff from non OGL sources, like gods and other IP)