To me there are several distinct possibilities.
One is Garrett is better than this fanbase thinks. I doubt there are any former players, coaches, management, or any employee of a professional team posting on this site. So we must consider that we just aren't educated enough to know what Garrett brings to the table. Especially since we see him three hours a week, sixteen weeks out of the year as he performs his duties as head coach.
The second idea is this. Jerry and Stephan have a grip on reality and understand they have never actually performed their job well enough to compose a team that can win it all after Jimmy Johnson left. So they see Garret can lead the team to a championship if they ever figure out how to assemble a team that has enough quality to get there.
Then there is the notion Jerry and Stephen are such control freaks guided by immense egos they cannot fathom hiring a coach with enough balls to tell them they are full of horse nuggets, and to run a successful championship team you have to do it a totally different way from drafting through coaching, while holding the players accountable and not in an environment that is like 53 Bedouin shieks in a tent filled with harem girls and anything goes. (run on sentences are your friend.)
How is the Irving debacle not an indictment of this whole franchise, as he flounders around, wasting immense talent, while the Three Stooges - Jerry, Curly, and Jas stumble through their jobs like homeless drunks that found the lost Maker's Mark truck and are not telling anyone?