[Macha]

> The OGL was written before VTTs (virtual tabletops) were really a thing, so it doesn't explicitly authorize them. Instead, VTT developers arrange their own deal with Wizards directly.

To expand on this about this, nothing around the existing v1.0a actually had field of use restrictions that would prevent its use in a VTT, and the accompanying contemporaneous FAQ indicated that using it for software was permissible. While VTTs as they exist today didn't exist when they wrote the license, character builders and video game adaptations did, so it's not like some VTTs are some totally unforeseen and therefore not plausibly covered.

So most VTTs are relying on the OGL where they just implement D&D and basic classes etc., the ones with deals are the ones that use additional non-OGL content which is why you can e.g. buy Curse of Strahd for roll20 or fantasy grounds despite Strahd not being OGL.