Co-op Adventuring in Dungeons & Dragons
When I started playing RPGs all the way back in the early 1980s, I did not have a group of players at my age to play games with (well, at least none that I ever found). Hence, I subjected my brother to Traveller and Star Frontiers — and eventually Marvel Super Heroes, Twilight: 2000, and others. RPGs had always presumed that the game would have a game master (GM) — sometimes called Dungeon Master, referee, storyteller, keeper, and others — and the players.
The GM is largely responsible for crafting the story, running all the non-player characters (NPCs), adjudicating the rules, and responding to player decisions by adjusting the story as necessary. Hence, I acted as the GM and my brother played the characters in the story. Typically, far more players are looking for GMs to run games than GMs hanging around without players. This is, of course, highly dependent on the games being run, location, and so on. With online gaming, it is usually more challenging to coordinate time zones.
That said, GMs may have trouble finding players to play anything but the giant of all RPGs, Dungeons & Dragons. However, D&D often serves as a gateway to other RPGs, and the incredible success of Cyberpunk Red and Aliens shows that as more players enter the hobby, a fair number are willing to expand beyond the D&D horizon.