Gaming

The term comes mostly because there is a upper limit(like you get to the last level and then it goes back to level 1), there is last boss from old RPG, you finished all 100 puzzles the designer put in place, etc. So it makes sense to say I beat the game cause there is nothing else the game can provide a challenge.

But modern games like you mentioned, aren't even designed with this approach. It becomes a wild variety of genre and flavor that you can pretty much do anything you like. ie. I finished GoW:Ragnarok on GMGoW difficulty 100% everything, while there are streamer doing Lv1/no damage runs, and there are people that play story mode and just want to enjoy the lore and story development. All are valid ways to finish the game. And there are even games, say Flight Simulator, how do you "beat" or "finish" that? Or competitive games like Rocket League, CS:GO those just never "finish", you get "better" and climb the ranks like online chess players.

However, you can still "beat" some games that are designed in more traditional fashion. Say, Resogun is a side scroll arcade shooter, it was traditionally designed so people tries to get better and chase high scores, but if you "beat" it as in finish the arcade mode, there is not much it has to offer than chase high scores. Or games like MetalSlug series, you beat the game cause they don't have any other challenges to offer.

Most people don't say "beat" the game now as we know how high the ceiling is that people can do with games(knife only runs, blindfold runs, rockband guitar controllers, etc.), we mostly say we finish the campaign, platinum the game, or 100% complete from the game stats(not every game tracks completion stats).