In the past 20 years of baseball, 22 out of 30 franchises have made it to the World Series. And of those eight teams that didn't make it, the Padres are the only team to not have any real shot of winning it all during that time frame, and even they made it to the WS in 1998. The other seven (Mariners, Athletics, Pirates, Blue Jays, Nationals, Brewers, and Indians) actually had a handful of quality playoff seasons where they were legit le contenders.
However, the NBA only has 15 out of 32 franchises making it to the NBA Finals during that time frame (The Western Conference has only produced five rofl). Even that is probably misleading since basketball is so much easier to predict an outcome than baseball due to:
a) not as much variation as in baseball due to the high level of difficulty. For example, a star player may strike out 8 times in the world series and go 1/19 at the plate but you won't see a star player with a shooting percentage that poor in the Finals
and
b) Star players having a much greater impact on the game in basketball than baseball as mid pointed out.
So even though 15 out of 32 teams reached the NBA Finals during that time frame, it's more likely that only 13 or so were actual contenders. i.e. it's probably disingenuous to call the 01 Sixers, 02-03 New Jersey Nets true contenders due to how ty the East was at the time.