Counter-Strike started as a free mod two college kids built on top of Half-Life. Valve bought it, hired them, and it became the best-selling PC shooter of all time. Anthropic just ran the same play on coding tools. The biggest franchises in gaming keep coming from mods. Dota began as a Warcraft III custom map, and that one map spawned Dota 2 plus the entire genre League of Legends dominates. PUBG grew out of Brendan Greene's Arma mods, and PUBG's explosion is the reason Fortnite built a battle…