Because he is batting at the bottom of the order. I am not sure what everyone is expecting out of the bottom of the order but nearly every single team that is competing has someone that is just horrible in their lineup. The Cardinals have Molina, Dodgers have Turner, Blue Jays have T. Hernandez, Rays have Zunino and Walls, Yankees have Hicks and Gallo and I can keep on going for some of these.
I just don't see CF being a need to trade assets when you are not going to get that big of an improvement. The current option of Taylor and Cain is perfectly fine and yes that means a team competing for a playoff spot. Once Adames and Renfroe come back CF is not an issue at all. As long as the defense stays strong in CF offensively it is fine to have that as a black hole offensively as long as Cain is hitting 8th or 9th there is no issue.