According to JustJared.com,
In a recent interview with TIME about Beautiful Boy, Steve revealed why he has no plans of ever reviving his role as boss Michael Scott in The Office.
“I’ve never thought of it as a good idea,” Steve expressed. “I think it existed in that time and with those people and it felt right. There was something so special about it and I guess it’s an odd way to put it but I love it too much to ever want to do it again. It’s too special to me in my heart in that period of time.”
I think he's right though. They gave us many of great seasons and there's no reason to revive something that was so special.