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.
S&box has rewritten many parts of the engine in C# at this point. Gets rid of lots of legacy code and makes this easier to change in the future. Using the scripting facilities built into Source 2 would have restricted what could be written in C# and slowed the team down.
Garry has publicly expressed regret over choosing Source 2 for this reason, but they might be in too deep now. Generally, the tooling is really great, so I guess that's the primary benefit.
Do you have a source for this? S&box used to use UE4 in the past but as soon as Source 2 became an option immediately jumped ship. There has been a ton of progress since then and I don't see why he would regret switching to Source 2 when no other engine would have made as good of a base for S&box.
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.