His time of possession per touch is 3-5% longer than Pritchard and White, the two players he backs up. It is a little slower, but not much. It is also a relic of earlier in the season for when he was learning his role on the team and his teammates. In the last 15 games he now has fewer second per touch than Pritchard and White.
Simons is one of four players who average 4 seconds per touch (the others being Pritchard, White, and Brown). No other regular hits 2 seconds, so it is easy to see that Simons role is supposed to involve more isolation, as is the case for the other three as well. Isolation plays also help create openings for other players. By being able to score one-on-one, Simons is more likely to draw at least some attention from a second defender, which leads to the passing you would like more of. He also is less likely to play with multiple other players who can create their own shot than the starters are.
The Celtics offense is highly efficient in general and is virtually the same with him playing as not, so I think this is more of a you thing.