A lot of it is because of how the champs are presented in MLB and other sports. In every sport, the eventual champion is always crowned the best, the most dominant, etc etc. Fans are constantly fed the narratives that the post season tournaments and their SSS results are more important when judging a team. Winning titles is what gives fans erections, so its a natural thing to believe that your erection is caused by the best team. Metaphorically speaking.
Probably a slight majority of the time these days, that is actually correct. But sports have changed in the past half century and especially in the past 20 years. Parity in most leagues is at an all time high (maybe with the exception of the NBA?) and especially in baseball, the differences in the top pitchers, top 9 position players, top 3 relievers from a 100 win team, to a 90 win team, to even an 84 win team are not so massive that you can reasonably expect "upsets" every single year with regularity in the playoffs due to the nature of how the rosters get artificially restricted with all the off days.