Nick Masset sucking and Dusty Baker not helping

I'm sure everyone is wondering why in the world, does Dusty Baker keep sending Masset out there.
Well it turns out, Dusty had no choice but to send him out there after Chapman didn't look so good. And we find out today that Chapman has shoulder inflammation.

So here is my question, why does managers yank pitchers who are doing well. Why burn a perfectly effective Jordan Smith on one single batter when you know you don't want to go to Massett. Why is it so bad to keep Jordan Smith out there for a bit longer?

That's one thing about baseball I don't understand, these managers have these unwritten rules that makes no sense. If a guy can pitch and if he is being effective, keep him out there. Do put your self in a hole where you have to bring in a guy that you don't have confidence in.