My cousins who are in their late 20s probably spend more time watching playthroughs than playing the games themselves. I think part of it is they simply can't afford to buy all the games they want to play. Another part is some games are just a good story that can actually be viewed and experienced through a playthrough.
Personally, if the game is easily available to me, I'd prefer to just play it.
Availability is the thing though. Some games could be really demanding, or have a huge install requirements. Like I'm certainly never going to buy a console and a copy of the last of us to play it, that's something I can just watch the cutscenes for. But then there was the new Zelda game and I refused to watch anything on it and only play it blind, knowing that watching people play the story would just worsen my experience lol. I'd say most games you can just watch to get the "gist" of it anyways. Also playing games uses more mental energy then just simply watching it.