Their problem is that they can't play defense. It's not that they don't want to, or don't have a good defensive coach, it's just that they physically can't. Nash is too old and too slow, Kobe doesn't have the energy, Dwight is obviously still injured he can barely jump, Gasol has mental issues, and Artest isn't consistent enough.
Older experienced teams used to dominate the NBA, but now that you have teams Like Miami, Oklahoma, Clippers, that have a bunch of young stars, experience isn't enough. The Spurs are a great example, they have had the same core for years, and their chemistry together gets them a great seed. But by the time they get to the playoffs they tire out. The Lakers might have had a better record right now if they had some chemistry, but they still wouldn't be able to last in the playoffs.