Compared to the one-and-done format of the NCAA tournament and the NFL playoffs, the NBA's best-of-seven series are designed to reward the better team, not the more fortunate one on a given day. Even the NHL, which also uses the best-of-seven format (along with MLB's League Championship Series and World Series) sees the better team win less frequently because the role of randomness is larger in a single game in those sports than the NBA.
But there's a danger of the notion that the better team wins an NBA playoff series descending to circular logic. Who wins a series? The better team. How do we know they're the better team? Because they won the series.
We know that isn't always the case. True upsets happen, even in a best-of-seven. But how often?