They KNOW there is no case.
They are keeping the charges festering intentionally.
It is leverage to negotiate a deal with the defendants so they drop their civil rights violations claims. Happens more times than people care to admit. (reporters don't cover the small cases where it happens because they don't want to lose access. So reporters keep the dirty secrets. see mark foley and the miami herald)
They are not dense, they are playing the real world game of prosecutors.