It's probably because believers have adopted and swallowed the basic Christian dogma without thinking through what the concepts mean. Of course that is what happens when children are indoctrinated in religious belief well before their brains have developed enough for critical thought.Why do you think they have a hard time explaining exactly what is sinful?
How many of the average Christian that you will meet on a sidewalk in Anywhere, USA will know any of that? They will likely recite a few of the 10 Commandments (Jewish rules), but beyond that? It's a theological buffet of anything goes. Heck, even the Southern Baptists said slavery was moral.The incredible and intricate detail in Thomistic moral manuals for instance makes it seem to me that they can be incredibly precise.