Cause selling new games is more profitable.

If a new games costs €60 and older games €5 or less (which would be a lot less on streaming services), they'd have to sell at least 12 old games for every new game they sell less cause of this change. And if gamers spend more time on older games, it's highly possible that they'd buy, even just a single game, less.

It's the same with movies or TV. They would only loose money if they make the whole archive available as there is just so much of it that some of the new things could become irrelevant.

Not that I'm against archiving, but it is caused by the creative sector having to have to make money, which isn't easy for smaller players, and greed.