I would say that engine code doesn't have too much value without expertise to go with it. It's massively complex system with many moving parts and clearly they are also struggling to get it to work. Now try to take game out of it and with same effort you might just write a new one... Without legal hassle or baggage of existing games.

For example some devs told that HL: Alyx has some hacks left-over from HL2... And I don't think CdProjekt is any better...