Today's NBA generally has three different types of teams. If you're not a team with a multitude of star players or a rebuilding team relying on rookies, then you've usually got one main guy doing a lot of the work.
Teams like the Houston Rockets and the Cleveland Cavaliers are obvious candidates. There's a single player who is doing the plurality of the scoring, grabbing rebounds, dishing out assists and altogether having very good games, but their teams still end up winning.
In James Harden's case, he has become the only player on his team that can be trusted at extended stretches to actually score, and if he doesn't then the entire game might as well be flushed down the toilet.
That's a big reason why this team is going to be very volatile over the course of the season, and if they don't find a consistent offensive player to go to besides Harden, it's going to be tough for this team to make a playoff run.
There are quite a few teams like that around the league that need to give their main guys a little bit of help, but for some teams, it's going to be easier than others.