Then how do we account for the fact that so few teams have actually won the whole thing?
The whole single-elim vs playoff series thing that has been repeated in this thread over and over. If the NFL didn't (by necessity) have single-elimination playoffs, the Steelers and Patriots would look as dominant as the Lakers and Spurs in recent history.
Yet if it were just single elimination making the difference, don't you think there would be more NBA champions if 23 different teams have made the conference finals in the last 17 seasons, considering that the NFL has had 24 different teams make the conference championship and many different champions?
In other words, if the number of teams in either sport that have made the conference finals in that span is so close, why is there such a disparity in champions?
Single-elimination increases the effect of luck on the outcome, giving the inferior team a better chance of winning. The NBA has a playoff system where superior skill is more likely to win in the long run. Given its playoff system, the NFL should see more variation in champions even if it has roughly the same distribution of skill levels that the NBA has.
One explanation for the disparity is that the Lakers and Spurs have been really, really good at winning when the make it to the end, while the Steelers and Patriots haven't.
If New England and Pittsburgh had been a collective 7-4 instead of 5-4 in the Super Bowl and teams like the Nets and Jazz had pulled it out in one of their multiple trips to the NBA Finals, maybe this wouldn't be such a talking point, so the troubles of small sample size is another explanation.
Some people tend to over-exaggerate the lack of competitive imbalance because they don't understand statistics and instead rely on anecdote-based analysis. Once you dive into the numbers, you start to realize that sometimes perception and "common" sense don't match reality.