You'd be right, in theory, if publishers "paid it forward" more often. But we saw with the Black flag SRD, and often with others throughout 5e's existance, that the third party rarely does. They open things required by the license and keep everything else as product identity. Under this usage, the OGL is a CC-BY license with baggage.
You can argue under ideal altruism that the OGL would be preferable, as it promotes more sharing. Unfortunately, we don't seem to live in that world.
I'm not sure what you mean regarding the Black Flag SRD, since as I recall that was an ORC License product and not an OGL one. Likewise, the entire point of the OGL is that the "required by the license" category is set in the license itself, covering all derivative materials (in essence, all game mechanics) rather than whatever the publisher does (or doesn't) feel like (even if there have been some bad actors claiming that Product Identity covers things that it clearly doesn't).
But really, the "ideal altruism" strikes me as very important, because even if we don't live in that world
today, we might very well
tomorrow.
Tabletop role-playing games have long struck me as having value in their
potentiality more than any other medium. I've seen numerous gamers buying products, not for any immediate value that they'd derive from them, but instead finding value because they think they
might be able to use them
someday. "A desert sourcebook?" says a gamer, currently running a political game in a temperate climate, "I should pick this up in case the PCs get suckered in by that red herring the ambassador threw them last week about going to the Land of Burning Sands." Or maybe they pick up a book about lycanthropes because they think it might be an interesting twist to have the ambassador be a wererat, and could give some interesting tools to work with. Or that some new monster book might have just the thing for a summoned minion, etc.
Heck, half the time they don't even worry about issues of compatibility; the ideas
alone are worth mining, particularly if it's something that is easily (for them) converted between game lines.
Open Game Content leans in to this idea of potentiality. It leans into it
hard. It allows for content creators to potentially find something that's perfect for what they're doing years, maybe decades, after it was published, available to be used and ready to be expanded upon. Does that happen very much? No, but the possibility is there, and in that regard strikes me as evoking the spirit of tabletop RPGs in a way that I find to be evocative in the extreme, and which the CC-BY-4.0 just doesn't live up to compared to the OGL.