Nothing. You can also make an iPhone-compatible phone case and a wheel nut which fits Ford cars. Many do.If I decided to make a 4E or Cyberpunk Red or WoD compatible adventure, what is stopping me? (Assuming I am not violating any trademarks.)
They give you large swathes of text you can literally reproduce (as in copy/paste). You can use their text verbatim, en-masse.You don't need to reproduce any copyrighted text to produce a compatible adventure or supplement for any game, so why do SRDs and licenses exist?
But you don’t need to be able to copy/paste several pages of the D&D rulebooks to make compatible products.
As I said, the exception might be a VTT or character builder which literally does copy/paste entire chapters, but adds functionality, but to make an adventure? A new monster? A feat? A class? A magic item? New rules? Nope. Those are your words.
(The secret weapon was always the compatibility logo—the d20 logo back in the day, whatever the current Pathfinder compatibility logo is, etc. These days every other compatible 5E product on Kickstarter literally just copies WotC’s trade dress and they don’t seem bothered about that, so a compatibility logo is no longer needed, but that’s an entirely different conversation).