If the Celtics were to have enough cap room after acquiring Cousins, they could do so. However, because he only has six year's experience, he is not eligible to make the same amount as Harden, but rather the same amount as Hassan Whiteside, which is considerably less. So there's no reason for him to do so.
Griffin has a trade kicker that will take him to near the max. While he'd be eligible for an extension, you could only give him a $2 million raise because he'd be at the max with more. He's not going to give up hitting free agency for that.