Sneakylong
The NFL is a business. Good businesses make good decisions. When you look at Dallas and the Giants, you have one HUGE thing in common - owners who are ultimately really shitty managers. They don't know what they're doing with personnel. They're playing checkers while the good businesses they're primarily competing against are playing chess.
I thought letting go of Zach Ertz a few years back was a horrible decision. Turns out I would be a shitty business manager, because it was precisely the right move. Knowing when to move on, knowing who to pick up in free agency, is such an understated aspect of success in the NFL.
Simply put, some teams get lucky. The rest just understand the business part of the NFL better than others.