S&box licensing Source 2 is a bit of an enigma to me, I get the impression that they're having to rewrite huge swaths of the engine so I don't know what they're getting out of it. They've gone to the lengths of integrating C# scripting from scratch rather than using the scripting facilities that Source 2 comes with. Nobody else has licensed Source 2 in the 4 years since s&box adopted it, so they're the lone outlier in any case.

Respawns use of Source 1 was a similar story, they ended up rewriting practically everything in the course of developing Titanfall and Apex Legends, to the point that it's almost unrecognisable as Source at this point.