Yeah, it's not that companies "forgot" how to design for offline use.

They deliberately chose to design against it, because they discovered they can extract a lot more money by offering paid Progression Boosts so that players can more-quickly acquire Artificially Scarce Reward, or selling additional draws of a virtual lottery.

So it's a business thing. In terms of technology, it's the opposite: Offline play is more feasible for more kinds of games than ever before, since local AI (of various types) has improved, reducing the need to recruit a human counterpart over the internet.