Because if they face the truth it means they are inferior.
It's disturbing to me how true this is. You know those people at work who aren't really dumb but aren't that bright either, who just keep doing things the way they have always done them because that's the way they have always done them? They've been around a long time and they know the details of the systems/products/whatever they work with so they're pretty effective at their jobs, but they aren't deep thinkers or innovators or anything more than a cog in the machine.
As best I can tell, that's almost every coach in the league for the last 25 years.
They've had some success due primarily to experience rather than actual intelligence and because they work hard at doing what they have always done, they assume they're working as hard and as smart as everyone else when they aren't.
The perfect example is the end of last year's Super Bowl. I don't remember all the details, but Seattle mismanaged the clock so they had to take a time out to avoid a penalty which meant they couldn't stop the clock after a running play which meant they had to throw the ball if they were going to get four chances.
Meanwhile all the coaches on the New England sideline knew the Seattle formation meant pass, and got Butler into the game, and Butler--a backup going into the game--recognized the play because he had practiced it that week, and knew where he had to be to make the play.
Then with the line of scrimmage so close to the end zone every single one of us was worried about a safety even on a safe run-into-the-line-and-fall-down play, the Seattle players were undisciplined enough to commit a penalty and give the Pats five yards and make the safety much less likely.
A two time super-bowl coach with one of the best teams in the league mismanages the clock and hasn't instilled enough discipline in his team for them not to commit stupid penalties in the last seconds of the championship game.
The other guy not only had someone identify the plays Seattle used in those situations, but had his backup cornerback practice them.
Belichick is just better at this. He's better at hiring coaches. He's better at picking players. He's better at preparing players. He's actually smart and competent and willing to do something new if he's convinced it's what it will take to win the game and he's competing against people who are just good at doing what they have always done. When he has good players he's going to have a good team. When he has one of the best players ever at the most important position in the game he's going to have a level of success that other coaches can't even dream of.
But because the other coaches, players, and management are limited in their mindset, they can't see that he actually is better than them and think it must be some combination of cheating and luck.
The Patriots didn't start winning Super Bowls until 2001 and are two miracle catches away from being tied for the most super bowl wins ever with a team that started winning them in 1974.