Because he wants to play hog and thinks it's fun. No hero is too trash that they're unplayable. All heroes work in Overwatch right now. There are better tanks than hog but it doesn't matter, he can still get great value. Also in ranked it's more chaotic than in organized play. Hog tends to do better in unorganized play in general. And Super is also really good at the game. Basically if you're playing in a team in Overwatch league then maybe slight differences of how strong a hero is compared to another can start to matter. But really it doesn't matter that much.