PS+ set forward a theme of letting people have game trials - you can download and play for a few hours before needing to buy. I think they want that tied to some kind of invested subscription setup just so that people wouldn’t abuse the system.
It’s easier to avoid abuse if every game has demos coded to end after level 1, but as many old analyses have shown, that takes a huge amount of developer resources.
The issue of downloading 100 GB is something that some publishers have tried to solve with cloud gaming. If you’re only mildly interested in a Game Pass game, you can play it on cloud, and then if you enjoyed your first session, download it locally for the next one.