The regular season games are sometimes 3 or 4 months before. Teams change, injuries happen, game plans get figured out, tons of adjustments are made, things change. To say a team is better because they won a few weeks or months before doesn't make any sense.
You keep going on and on about the 2007 Cowboys, but did you look at anything besides their final record? They won every single game in their first 13 weeks except for a loss to NE, and no shame in that. But in the last 3 weeks, the offense that averaged over 28 points per game was held to 26 points TOTAL on their way to losing 2 of 3 games, scoring 6 points in each of those losses. The only win was a 7-point victory over a 7-9 Carolina team that got outscored by 80 points in the regular season. Clearly, the Cowboys were not playing well before they even got to the play-offs.
Those Cowboys played the Giants opening week, and anything can happen in the opening week, so that game doesn't really mean much. Heck, we lost to Buffalo 31-0 in the first week of a season and went on to win the Superbowl.
The next match-up was November 11, but then over two months would pass before they would meet up again. Teams change quite a bit over two months. The Bills won 5 games in the first two months of this season, then would take another two months to win their 6th game of the season.
Maybe the 13-3 Packers deserved it more. They were the better team. Then again, they lost both division games to the 7-9 Chicago Bears who didn't even make the play-offs. Does that mean Chicago deserved a shot instead? I mean they beat the #1 conference seed twice.
There is always the chance of someone having a bad day and losing, and the simplest way to eliminate luck would be to have more than one game, say best 2 of 3 advances. Yet we see it in other sports that do have those systems where the "best" team doesn't always win in a multi-game series.
There are always upsets. Life is filled with upsets. But while you seem obsessed with the outcome of that one Cowboys/Giants game in the play-offs, just remember the Giants still went 10-6 in a division that sent 3 teams to the play-offs. They had a better record than 9-7 Tampa, yet had to play there in the first round because Tampa won a weak division. They beat the top two seeds in the conference at their places. Then they beat an undefeated Patriots team. You can cry fluke all you want, but the overall record over the last 4 or 5 games is hard to argue.