The OGL specifically protects use of the copyrightable text of the mechanics, that's the whole point of the OGL-compatible System Reference Document. Anyone can

The OGL specifically protects use of the copyrightable text of the mechanics, that's the whole point of the OGL-compatible System Reference Document. Anyone can write a d20 system with the same mechanics, but the more literal text you crib from the SRD, the more you expose yourself to litigation — unless you agree to the OGL. The OGL says, here, crib from and directly reference this copyrighted text so you can eliminate broad swaths of reinventing the wheel and focus on your compatible content. Third-party publishers on shoestrings adopted it en masse for both litigation safety and convenience.