This logic is flawed, you are assigning equal probabilities to every team in the league. See, the chances a coin lands heads or tails is exactly 50%, there is no tendency for the coin to favor a head or a tail (unless it is weighted).
You cant do that with professional sports teams because the probability of winning a SB is not the same for each team, you need to use weighted probabilites to show that some outcomes are going to have a higher tendency than others.
For example, do you think the Colts with a rookie QB is going to have the exact same chance of winning the SB as the Patriots? Of course not, thus the Patriots must be given a "higher weighting" in the probability tree.
I hated stats class in grad school, but I still remember a couple of things.