There's a limited number of plays in an NFL game, and an even more limited number of plays that actually have an impact on the final score. These includes fumbles, and who picks up a fumble is entirely random. Other sports get around this randomness but smoothing it out across a 7-game series. The regular season of the NFL somewhat smooths this out across 17 games, but even then the randomness of a single game or even a single play (or single penalty) can dictate an entire season's outcome (missing out on a bye, missing out on the playoffs, etc.)
But whether you win or lose in the playoffs hinges on a small number of plays, which means that the "better" team wins only a tiny percentage of the time more than the "worse" team. Seattle was a great team but they very well could have lost to the Niners if Michael Bennett does not recover a Kaepernick fumble early in the 4th quarter. Or if a Russell Wilson fumble in the 4th quarter does not bounce right into his hands. The Super Bowl is probably an entirely different game if the center does not snap the ball five feet over Manning's head on the first play from scrimmage. And so on.
Building a team in the NFL is about building a team to win the most number of games in the regular season that you possibly can to get the best seat at the playoff roulette table with the most chips that you can. But even with the best seat and the most chips, you're still at the mercy of the wheel.