Because, within the realm of morality, we DO have option to make decisions as we wish, culturally, without having to try and go against the grain of something that simply will not have it. In the area of bridge building, or even Civil Engineering as a whole, the moment you decide to go against the grain and try to build something that flies in the face of the physical laws to which load bearing members adhere, you've made the decision to see your bridge collapse. There is a definitive point, and definitive criteria of judgment by which someone can state "you are doing this wrong." e.g. the bridge you built collapses, or reacts to vibrations by turning itself into a violent sine wave.
But, culturally there are all sorts of differences in supposed "right and wrong" that are "working" to greater or lesser extent between every two cultures you might hold up for scrutiny. To say there is an end-all-beat-all "right" way to go about establishing moral rectitude is just... well... it's strange, to tell you the truth. With morality we aren't reacting to immovable forces like gravity and finding the best-case scenarios that get us where we want to be. As a race, humans have variously decided what to react to and what not to when it comes to morality, and you need only ask two people with differences if they think their principles are "the right" principles, and receive two answers of "yes", to realize that calling either one "wrong" isn't going to get you very far. They'll just say "prove it.", and you're left with absolutely nothing.