Coaches absolutely deserve credit, blame, or even firing based on the aggregate. But let's be honest, they never touch the football.
They get paid, and so the easy answer is always to assign 100% credit or 100% blame to an individual that never touches the football. It's a nice outlet for fans.
But Bronco doesn't deserve 100% credit for any of the 99 games they won. And he doesn't deserve 100% blame for any of the games they lost.
It's a team game. Probably the most team-centric game in any sport. The head coach and the quarterback are by far the most important pieces.
But you can't play a game if those are the only two people who show up.