But if you look at the other major sports leagues not only can you find multiple examples of those level of teams playing for the title, you can find them winning the title.
Although I agree with your general point, I think you are exaggerating. Only 8 MLB teams make the postseason, so no, you've never seen a 10th or 11th best team play for the title. As for the NFL, wild card teams do sometimes get to the Super Bowl and even win, but the difference there is that the playoffs are single elimination, making upsets much more likely than in seven-game series. Not only that, but the short NFL season (in terms of number of games), as well as the imbalance in any given team's schedule, means that there is much more uncertainty about who the best teams even
are, in spite of what the authors of "power rankings" will tell you.
The main reasons that the NBA playoffs seem so predictable are that (a) it's very likely that the higher seed is indeed the superior team, and (b) the seven-game format makes upsets statistically very improbable.
