I just have no idea why he would leave just yet. Even an honest coach can't tell you he isn't looking ahead to next season and what returns and how the schedule will play out and not think this team could have a legit playoff chance in 2019. Who would walk away from that? This is all him, he is the CEO/GM of this team. He has no one to tell him how to run his team. If he walked away, I think he would be shunning himself with what ifs in the future.
I still think the problem is that we think of this in the context of typical, non-D1 football coach people. To become as successful as Campbell you have to be wired a little different. Ultra-competitive, extreme confidence in yourself, etc. You don't get to that point if you aren't. So while many of us can't see the purpose in giving up a multi-million dollar job with great job security for another multi-million dollar job with a great deal more stress and expectations, we also don't think quite like Coach Campbell does.