IMO the source engine codebase is probably chock-full of duct tape and cruft, full of undocumented, legacy, bespoke, hacky and deprecated stuff that it's not worth the dev resources for valve to bring it up to an OSS standard worthy of their reputation.

Contributing to this is probably

- custom external hooks (eg: homemade test framework, patchnotes publishing, steamapp backdoor integrations, hardware-specific firmware interfaces, 3rd party closed source SDK hooks)

- assumptions about Valve's server architecture/implementation for most multiplayer stuff used by Valve games, the codebase(s) of which are probably as vast as Source itself and closed-source too

- bespoke engine modifications made for specific games like HL2 or CS1.6 which hasn't been touched for a decade, the authors of which may not be accessible to document them trivially

Adding sufficient documentation to a massive closed-source system meant for internal use, over multiple decades, to bring them up to par for functional OSS publication is a monumental feat that honestly probably isn't worth the risk of bad publicity from the modder community who'd just be mad about how unusable it would be.