Most people watch big tournaments and/or high level competitive players (LoL, Dota2, cs:go, SC2). LoL and Dota2 run tournaments with prize money in the millions

Most people watch big tournaments and/or high level competitive players (LoL, Dota2, cs:go, SC2). LoL and Dota2 run tournaments with prize money in the millions and the competing teams are fulltime players that live together and have coaches and sponsors, etc. Imagine Messi (or some American star athlete, eg. LeBron) streaming his training via a GoPro. Fans would be very interested. Single player / non-competitive games (such as your example, Fallout 3) are indeed much less exciting and get less viewers. There are also streamers that aren't very good at playing anything, but they're entertaining in one way or another and have built up a following that just watched them play whatever.