I’m guessing Steam decided against being able to leave your games to somebody else when you die because of how most EULAs I’ve read work: they are often non-transferrable licence and so in most cases the store has no choice in the matter. Now GOG are willing to say they will do what they can given this limitation, but I can see why Steam wouldn’t: it’s a whole lot of work for realistically not much benefit. It’s probably easier for Valve to gift the same games over to the new person.
Last time they’ll ever do that! Pass the buck of hosting web-facing Plex servers onto somebody else.