I think often he just has too much on his plate to be closely involved with the adaptations - he mostly has two new books out a year. People like Rowling or Martin are far closer involved with their adaptations, it appears. He has so many books out and is constantly writing new ones that he doesn't seem to spend that much time on sets and in meetings.
Also, as that last article said, he isn't so eager for them to be filmed. And therein may lie the answer to the question of this thread: in the end he doesn't seem to care all that much. He wants to allow everyone to film something he wrote, but he doesn't closely guard it. In a way, it is strange because he is a big filmfan as well as a bookfan: his books constantly mention filmtitles and he does these 'best film of the year' lists.
It may be just the multitude of his works that he can't control them all and chooses a more wait-and-see attitude.