Because Goyer was reported to have already started writing Batman vs Superman before Man of Steel released. Terrio was originally reported to be coming in to polish Goyer's script. Since then, Terrio has been given the screenplay credit and Goyer relegated to a story credit - which suggests that Terrio's changes to Goyer's script were substantial enough to change the credit due.
Ultimately though, he's still working from Goyer's original skeleton.
And? Plenty of people have polished Goyers scripts before and those movies turned out fine. I'm not sure why this movie in particular is the exception as reason to shit on it.