Isn't that because a lot of GoldSrc was idTech-derived enough that the legality of open sourcing it is trapped in contract law limbo? Even though those years of the idTech engine itself are now also open source, the contracts at the time did not plan for that and it is likely at this point that solving those contracts would be a 3-way legal question between Microsoft (ActiVision because of Vivendi/Sierra, Half-Life's original publisher), Microsoft (Bethesda because of inheriting idTech), and Valve, with the obviously problem in the way of that Valve and Microsoft have a complex history and aren't likely to want to get into a legal discussion if they can help it.
I seem to recall a fan project trying to take idTech's open source and recreate GoldSrc's fork from it by trying to reverse engineer from the parts of Half Life that are open source but not having much luck because the divergence was strong enough in some places to be somewhat impenetrable without some other Rosetta Stone.
Maybe.. kind of.
The Doom source code was originally released under a non-commercial license that was weirdly restrictive and it was eventually re-released under GPL. The Quake source code was released under GPL from the beginning.
If Valve really wanted to release HL1/GoldSrc source code, they could re-base to the GPL quake source code and release their changes as GPL as well. This would be a miserable job because the remaining quake code is probably scattered across the codebase in weird orphaned fragments, but afaik it would be completely legal.
e: oh yeah if the shambling zombie that is Sierra still holds any rights over HL1 then god knows what the IP situation is with that property