Not every team starts its best 5. Sometimes a team will bring a better player off the bench because of better team fit, like Manu, but that's uncommon. In most cases, the best fit for the best lineup is the most talented player and the coach is going to play his best lineup at the start of a game.
Why? When a visiting team takes an early lead, it often keeps the crown out of the game. You don't want to fall behind on the road. The visiting team is going to be playing their best lineup to start the game.
In the third, letting a weaker team linger often leads to hard fought 4th quarters. Also, if a team rests its best players at the beginning of the third and then plays them the next 12 minutes, how tired will they be in the last 6 minutes of the game if the score is still close? I think we can all agree that the last 6 minutes of the game are frequently the most important part of an NBA game..