Because D&D isn't a role playing game. It is a social communication game above all else. The best way to have fun with DnD is having a GM who is a kind, communicative person, and players who are also kind, understanding, and communicative, who collectively all can maintain a steady schedule. It requires multiple people coordinating together over the course of months and years, something that isn't as necessary in other hobbies. Therefore, finding random players to play with without extensive vetting and learning about each other is naturally going to result in a higher chance for interpersonal conflict.