If I understand correctly all American sports are like this. How do they keep it interesting?
As the great Ceferin would say, the love of the game. Players are competitive, so they don't give up games, and teams are close to each others from top to bottom so unless a team is in tank mode even a game against the worst team is highly competitive and plays a role in the post season narrative, there is a point during regular season where you anticipate playoffs and wonder which teams can beat each others. To be honest it's significantly less boring than your average european league game, though it's true that the NBA and the MLB have so many games that you aren't really supposed to watch all of them but you really don't want to miss games against very good teams or teams that are in good form even if one of them isn't at the top.