You must first start with a fundamental truth about what Jerry looks for FIRST in a head coach:
COMFORT. Not the coach’s ability to win games (although that would be nice), not the coach’s ability to prepare or adjust, not their ability to draft and develop talent (JG is actually pretty good at that). With Jerry, COMFORT IS KING.
Jerry first wants to feel like his HC is a family member- a son that he can say or do anything with and not have to worry about silly NFL protocols like the HC picks his own staff, or the HC has control of the locker room, who starts and sits, and that the buck stops with him regarding player complaints or concerns. Jerry wants the comfort to do whatever he wants, whenever he wants without any objection.
Jerry does not want a HC who has the power of personality to compete with him for the “almighty credit” should his team ever win a SB again. Jason Garrett has strengths, and he’s certainly not a complete disaster. But he also is not anything like most NFL HCs who command great respect among NFL observers.
Garrett is certainly not the worst HC in the NFL. But with the most valuable and visible sports franchise in America, Jerry could certainly have a more talented and successful head coach. He has the money to outbid anyone for the best. But that’s not what he wants.
He values COMFORT above all else.