You can't just throw the same 2-3 guys out there every night. A manager has to be conscious of who they're facing, who they will face tomorrow, the day after, and who they're playing next week. They have to see what matchups are going to happen or could happen. Guys like Garza, Jacques, Ort, Kluber (he's on the IL now) are going to have to pitch at some point, and unfortunately, the perfect opportunity (up 15-1 or down 15-1) is not going to present itself as much as it would to make the decision easy.
Assuming you have 5 guys in the rotation, a manager only has 8 guys in the pen, and he's going to need 3-4 innings every night. If you have guys on your roster sitting and not pitching, then the fewer innings they pitch are going to be made up by the other guys thus increasing the chance of injuring the good guys who are actually worth something.
I get in a vacuum, looking at one game it can become easy to scratch our heads at Cora "Why did he bring this guy in" but the reality is, he has to think about the rest of the week and all 162 games, and keeping guys healthy.
It's not as easy as it looks from the armchair.