That really is the majority of NBA teams, though. Off the top of my head, in the last 25-28 years, the only teams to win the title have been the Celtics, Lakers, Pistons, Bulls, Rockets, Spurs, Mavericks, and Heat. Maybe I forgot one, but I'm too lazy to do a 5 second google search right now. 8 teams out of 30 have won championships in almost 30 years. All but one has won multiple titles in that time. The list doesn't get much better when you add the finals losers (76ers, Sonics/Thunder, Jazz, Knicks, Magic...I'm sure I'm forgetting someone here that wasn't already on the winners list...oh, the Nets! Edit* Oh yeah...Cavs, too...). Maybe half the teams in the NBA have competed in the Finals in that timeframe?
Most of those winners are there because of transcendent talent. A couple are there because the front office managed to put together a strong, deep team. I'm not sure this makes you feel any better, but I'd rather watch the Wolves than the Hawks team that's been a 3-6 seed for the last several years.

If you're going to be mediocre, at least be entertaining. #lessonslearnedfromVinceCarter