The reasons why people watch other people play video games are several:
1. People like to see someone of a very high level play a game that they too play.
2. You often get to see someone playing a game that you never heard about. For instance: The recent speed running marathon streams have showed a lot of obscure and old games.
3. And most important: People are entertained by the commentary from the streamer.
If you just run a game with no commentary you will be hard pressed to get any viewers. It is more that with the younger generation games have become a huge part of culture and to see someone playing games you play as well and chatting give the viewer a social experience. There is chat too and the streamers often address people in chat. If you look at the most popular personalities on YouTube many of them are gaming related. People spend a lot of time playing today's very immersive, social, and large scale games so it follows that they can be entertained by personalities involved with them.
You're not the only one... a friend of me who works in the IT department for a big IT company says about 20% of their daily network traffic goes to Twitch nowadays. It's a bit ridiculous if you think about it but he said he has many colleagues who have a stream open on the background of one of their screens.
I think a cross between #1 and #3 is easily the lion's share of viewership, especially if the game is difficult and almost antagonistic toward the player. I'm reminded of the Youtube videos posted some years ago by ProtonJon of his Super Mario World ROM hack play-throughs. I'd never personally play them, but it's easy to get an almost schadenfreude-level of enjoyment out of watching someone else suffer through a game that's extremely punishing. The same easily applies to other games that most people wouldn't ordinarily play for a variety of reasons (too old, as in your example, or perhaps too frightening e.g. Amnesia--that one made the hair on my neck stand up).
1. People like to see someone of a very high level play a game that they too play.
2. You often get to see someone playing a game that you never heard about. For instance: The recent speed running marathon streams have showed a lot of obscure and old games.
3. And most important: People are entertained by the commentary from the streamer.
If you just run a game with no commentary you will be hard pressed to get any viewers. It is more that with the younger generation games have become a huge part of culture and to see someone playing games you play as well and chatting give the viewer a social experience. There is chat too and the streamers often address people in chat. If you look at the most popular personalities on YouTube many of them are gaming related. People spend a lot of time playing today's very immersive, social, and large scale games so it follows that they can be entertained by personalities involved with them.