There is a term called 'Abandonware', games/software made which is not maintained anymore. Actually at this point, Windows 1/2/3/3.11/95/98/2000/ME/Vista/XP and soon 7, is considered abandonware, no longer under the care or maintainance of the author. Quite simply, they don't care. I'm sure all of those have far greater importance than any 70's-90's games.

Games are also 'abandoned' because companies fell under, and there's no one legally who can sell or handle them. They simply cease to exist.

As for the games you listed, they are all pretty much abandonware, the only difference is some compatibility updates (not with source), and the IP's themselves, namely the name is the only thing that is reused with the memories and the old sales to pick them up. Even full remakes like the newer monkey island with old graphics options, is more for a niche audiance of those wanting to replay them, not newer users.

Seriously, how many games are coming out using the Wolfenstein 3D engine today? I'd be surprised if 1 or 2 are. Certainly in the day the engine was reused, and Noah's ark, captain crunch cereal and other games utilized the engines while replacing sprites and maps and effects to make their own game. Though i do remember a 3D games book that included a raw engine and sample game to help you make your own, with source included.

The real problem is the copyright law. It is too damn long, and shouldn't last beyond 30 years. Disney being a major culprit, and yet now look at them floundering. No imagination and throwing out talent to hire cheap labor, and putting all their eggs in Disney+ when that isn't what made the company (and is likely going to seal it's doom).